Tuesday, August 31, 2021

Ida's Remnants to Impact Region

 Hurricane Ida battered portions of the Gulf Coast (particularly Louisiana) over the weekend and into the early parts of the work week. While the actual storm system is no longer a hurricane (or even a tropical storm)...the threat to life and property will continue into portions of the Mid-Atlantic and Northeast. 

The most substantial and widespread threat from Ida's remnants will be the flooding risk. There is a very large area with a high risk of excessive rainfall outlined by the WPC. Rainfall amounts of 6+ inches (locally even higher) will occur. The biggest risk for flooding will lie well north and west of the DC/Baltimore corridor. However, some risk for flash flooding will still be present due to the tropical airmass. 

A high risk of excessive rainfall is in place in portions of Pennsylvania and into the NE US.

Regardless of the placement of the heaviest rain...many areas have been receiving significant rainfall over the past few weeks. This may lead to flooding even in areas that do not get the highest totals.

Significant river flooding will become likely in the regions that receive significant rainfall. Big rivers like the Potomac may take a day or two (or longer) to crest. Therefore, flooding will remain a risk even after the rainfall ends. 

The next threat will be from tornadoes. The shear profile and instability will be conducive for bands of storms to form in the warm sector. Unfortunately, these parameters may line up just right for an elevated threat of tornadoes centered in the DC/Baltimore area and then over to the Eastern Shore. 

While 10% may not sound like a big risk - these are severe weather probabilities based on the background probability of a tornado within 25 miles of a point. Therefore, a 10% risk represents a SUBSTANTIALLY higher than normal probability of a tornado. This also is equivalent to the "enhanced" category of tiers from the Storm Prediction Center (SPC). 

A risk for tornadoes will exist in portions of the Mid-Atlantic tomorrow.

Synoptic (non-thunderstorm) winds may also be fairly gusty tomorrow. Some places may gust to 30-40mph at times. Places with saturated ground could see downed trees and power lines even in the absences of damage from storms. 

Updates will be provided as needed - be sure you and your family has a severe weather action plan in place. 

Wednesday, February 17, 2021

Significant Winter Storm to Impact Area

Odds are increasing that a high impact winter storm will impact the region beginning early tomorrow morning and continuing to tomorrow evening. Uncertainty still exists with regards to the potential for mixed precipitation, as well as where the heaviest banding will setup. Nonetheless, most if not everyone in the region will see eventful winter weather...and some will see substantial impacts to travel and activities. 

Below, you'll see the 06z GFS model. As we get closer and closer to the event, I'd suggest weighing more heavily on the mesoscale models like the NAM, RGEM etc. 

The 06z GFS run from 2/17. This indicates a period of snow and then mixing for the DC area.

Next is the 06z NAM run followed by the 06z RGEM run (that is a Canadian regional model). 

The 06z NAM run from 2/17. Another possible progression to the storm system.

06z RGEM model (a Canadian produced model). 

At this point in time, I think it's safe to say that the favored areas to the north and west of the I-95 corridor will do the best in terms of snow totals. Sleet and freezing rain mix potential will cut down the snow totals closer to the metro area as well as to the south and east of the corridor. 

That said, the less snow any one location sees, the more sleet and icing will be possible. Areas near Fredericksburg and running into Southern Maryland have the potential for a SIGNIFICANT icing event after an initial burst of snow. In these areas, there's even the chance at icing accretion of in excess of a quarter inch. This could lead to power outages in areas hardest hit. 

Conditions may vary greatly over relatively short distances. Thus, your exact location will have large impacts to your exact forecast. I'll do my best to break it down here - apologies if your exact location is not mentioned. 

I-95 Corridor (including areas like Silver Spring, Laurel, and running up towards Baltimore)

2-4 inches of snowfall seems probable. After that, some accumulation of sleet (possibly significant) may occur to give a "crust" to the snow. Additionally, a little glaze (up to a tenth of an inch or so) will be possible to top it all off. A true, "kitchen sink" of precipitation types and NOT fun for driving or even walking. 

North and West Zones (including areas like Frederick, Westminster, Sugarloaf Mountain, Damascus etc - think places like Frederick and Carroll County in MD and Loudoun in VA)

4-8 inches of snowfall with isolated totals to 10 inches. This area may not see much mixing at all - but a little sleet is possible - especially the further south and east you get (closer to the first zone up above). This area should see a storm system more heavily weighted towards snow. Power outage risk should be much lower for these places.

South and East Zones (including areas like Fredericksburg, Charles County, Calvert/St. Mary's)

1-4 inches of snowfall with the most likely being in the 1-3 range. Sleet may become the predominant precipitation type pretty quickly in this area. Even then, a transition to freezing rain may occur much sooner than the other areas listed. This is dangerous as it will create the potential for a significant ice storm component. At least a tenth of an inch of ice, with the potential for four tenths (perhaps even a half inch in spots) will be possible here. This will be enough to cause potential power outages and MAJOR travel concerns. Folks in this area should at least PREPARE for the potential of outages. 


Of course, all of the forecast hinges on how deep and how far north the warm layer above the surface gets. Confidence is high that this storm will have a good supply of fresh cold air to work with at the surface. But there will be some warmer layers of the atmosphere up near the 700mb level that will be the cause of the mixed precip. If these layers are only thin and not too warm, sleet may be predominate in the mixing. However, if the warm layer is on the warmer or thicker side, freezing rain would become more probable. These are factors that are difficult to forecast even at shorter ranges. 

Stay tuned for any refinements or updates to the forecast. This should be a pretty significant/high-impact winter storm for the region. There's the potential for more "waves" behind this system. It's possible these will favor plain rain, though. 

Wednesday, February 10, 2021

Another Two Part Winter Event

The National Weather Service has posted a *Winter Weather Advisory* for most of the area for tonight into tomorrow. All indications are that a slug of snowfall will impact the area from this evening into tomorrow morning. That being said, most guidance has converged on lower snow totals than were seen as being possible earlier in the week. Even so, the Euro model still indicates a shot at low-end warning criteria. In general, I think the NWS call for 1-3 inches of snow tonight is solid. I think it could push into the 2-4 inch range in spots that get better snowfall rates. 

The second wave looks to miss the DC-Baltimore corridor to the south. However, it's not out of the range of possibilities for this to trend back north. This second part of the system would be from later on Thursday and into Friday. 

TL;DR - A general 1-3 inches for much of the area tonight into tomorrow. Roads may be dangerous for travel. Some areas could see a bit higher. The second wave is still uncertain but for now, looks to miss. 

Tuesday, February 9, 2021

Long Duration Winter Storm Possible

Confidence is high that winter weather will impact the region once again this week. Currently, it appears that snow will develop from west to east across the area on Wednesday afternoon or evening. There continues to be some model disagreement with regards to exact details. This storm system is actually two "waves," and one may become dominant while one reduce in intensity. For this reason, while the odds for winter precipitation are high, the confidence in the exact timing or amounts are less certain. 

The 06z NAM model run from early morning on 2/9. This shows the progression that this model thinks is possible for the winter weather event for Wed-Fri. (Source: Pivotal Weather)

The 06z RGEM model from early morning on 2/9. This shows another model's take on what may transpire with the storm system for Wed-Fri. (Source: Pivotal Weather)

The 06z GFS model run from early on 2/9. This is just another example of another forecast model trying to crunch the numbers to determine what may happen for our next winter storm. (Source: Pivotal Weather)

You can see from the above forecast model loops, that the timing and intensity varies from one model to the next. However, there is a good chance at 1" or more of snowfall areawide (with the usual exceptions perhaps being to the areas south and east of the metro corridor). I'd even go farther and say that the chance for 2" or more are also quite high. Beyond that, a lot will rely on how potent each of the waves turns out to be in reality. This could be yet another situation of a winter storm that has a slug of snow, a lull and then perhaps additional snowfall with wave two. 

There is a chance that this could be a 4-8" snowstorm when all is said and done. Some mixing will be possible to the south and east. 

Taking a blend of various models gives the area a very healthy snowstorm. For now...I'd forecast 2+ inches with this system and leave it at that until some additional clarity is given by short term forecasting. 

"National Blend of Models" product from Pivotal Weather. This is a blended map showing what a blend of various weather models thinks will occur. This *could* be on the high end IF the system becomes weaker than modeled.

Looking even further ahead...there appears to be winter storm threats (very low confidence forecast at this time) for the weekend and then again Tue/Wed of next week. There's some early indication that significant ice could be in play for the weekend system...but of course it is too early to say with any sort of certainty. 

The bottom line is that we should continue to see a parade of at least THREATS for winter storms. Whether these pan out remains to be seen. However, the pattern certainly supports continued attacks by Old Man Winter. I'll continue to keep you updated as each system gets closer in time. 

Saturday, February 6, 2021

Winter Storm Warning for Tonight

* Quick hitting winter storm to impact the area overnight *

Winter Storm Warnings are in effect for most of the DC/Baltimore area for tonight. Snowfall from a fast moving storm system will move into the area after midnight. By sunrise, most of the area should be seeing snowfall...and some could be heavy at times. Some rain and/or mixing is possible mainly south and east of the I-95 corridor. The snowfall will be ending and departing the area by the early afternoon. 

Due to the potential for a few heavy bands of snow...this system may drop between three and six inches in a narrow stripe in the metro area. Keep in mind...only small adjustments could shift this area of heavier snowfall north and west or east and south. 

Temperatures will be on the rise Sunday afternoon, so plowing and melting should allow roads to become quickly passable again. That being said, the snow will be heavy enough to cause significant travel problems on the roads during the morning. I'd suggest staying off the roads until the afternoon. 

No fancy maps this time, as this will be a quick post for a quick hitting storm. 

Looking ahead, the pattern will support additional chances for significant winter weather during the later part of the coming week. But we'll dive more into that as the potential gets closer in time. 

Monday, February 1, 2021

Storm Winding Down - What's Next? [VIDEO]

Note from Pulse Weather: The video that goes along with this video is included both here, and at the bottom of this post for your convenience. The details in the text are covered in the video edition as well.

What's Next for DC's Winter? VIDEO

The current storm is starting to wind down across the area. I'll describe that storm system as a bit of an odd one. This particular type of storm is classified as something called a "Miller B" storm. Miller Bs are characterized by a primary low pressure system heading into the Ohio Valley and then redeveloping or transferring to the coast. Most of the time, the Washington, DC area tends to get a little "screwed" by these types of storms as they often develop the coastal just a little too late. 

Areas along the Mason-Dixon Line were treated to some great wraparound snowfall this afternoon. Totals there have really ramped up. Totals locally in Montgomery County and the DC/Baltimore area have been more tempered. It was an odd storm that spread light wintry precip out in a span of a few full days. 

Snowfall totals as shown by a map from the National Weather Service in Sterling, Virginia (LWX). This image is courtesy of their website. Click for full size image. 

Snow showers may continue through the night and into tomorrow in spots. Some gusty winds may also be  a factor - particularly to the north and east. 

So...What's next? [NOTE: A video is included at the bottom of this video for those not wanting to read a wall of text]

1) Moderating temperatures later in the week: Later this week, in the wake of our current storm system, temperatures will be able to rebound a bit into the 40s. This will be ahead of the next weather maker over the coming weekend. 

2) Significant dump of Arctic air with the potential for an associated batch of snow squalls: During the coming weekend, a significant batch of frigid air will come out of Canada and move eastward to impact the area. There's a chance that a secondary front behind the initial one could produce snow squalls similar to the Valentine's Day snow squalls in 2015. This is discussed in more detail in the video below. 

Robust shot of Arctic air shown dumping out of Canada on the 18z GFS model run. (Source: TropicalTidbits) - Click on the image for the full size. 

3) Storm system for the 2/8 or 2/9 timeframe: After the dump of Arctic air, a potential storm system will develop in the southern half of the country. Depending on the exact evolution of various upper level features, this storm system could pose a threat for significant winter weather in the area. Of course, being seven days out increases the amount of uncertainty. 

4) Follow up storm system for the period around 2/10: Immediately in the wake of the potential storm system for 2/8-9...another storm system may develop in the Gulf Coast states and track along the eastern seaboard. Being 10 or so days out significantly decreases confidence in any one model solution at this time - but this could pose a threat for follow-up winter weather for the region. 

All of these potential winter weather threats are due in part to a very favorable upper air pattern. A favorable -NAO block is forecast to be in place near Greenland. Of course, details will be easier to forecast as we get closer, but the threat is there for long lead winter storms. Occasionally, in this type of pattern, confidence can be higher from longer leads due to blocking and pattern stability. We'll see if things are more clear than they were with this most recent winter storm. 

500mb pattern as depicted by the GFS Ensembles from the 18z run on 2/1/2021. (Source: TropicalTidbits) - Click on the image for the full size. 

The big area of positive height anomalies shown near Greenland is what is referred to as a -NAO. Negative anomalies in this area would be a +NAO. There are other indices to monitor as well. The PNA and AO are other factor. Perhaps a winter storm hunter "crash course" would be an interesting post for the future. 

Check out the video that goes along with this latest post!

Friday, January 29, 2021

Winter Set to Grab Headlines - Snow On the Way

Snow enthusiasts rejoice! After a long gap between the December dose of winter precipitation and now, we are poised to get a visit from Old Man Winter! Despite lowest sun angle and shortest day length occurring in December, many people forget that our true "peak" winter weather climatology is January/February. Check your calendar - this storm is no exception! 

Speaking with a bit of technical weather jargon - a trough in the atmosphere will approach the area from the west. Exactly how this transpires will determine how the weather down at the surface level will unfold. That's the super "easy" way to explain things. In reality, the nuances that will go into producing a snowstorm for the region are much more complicated. 

The models have seemingly come into better agreement in terms of there being a snow event. However, key differences remain (as usual) in the specifics. Specific small scale features will only be resolved closer to "game time" and some features like heavy snow bands are always narrow and will potentially mean the difference between one location receiving a few inches and another receiving double-digit snowfall. 

The timing of the storm system is expected to come in two parts. The first part, which should be decent for the entire area, will be a slug of snowfall that will arrive Sunday between the very early hours and 7am or so (depending on exactly where you are). This will arrive from southwest to northeast. This portion of the storm appears to be fairly well agreed upon on most guidance and may drop between two and six inches. 

At that point, there may be a tapering off or lull in the storm. During this time, there is a chance that areas south and east (and even perhaps into the close in suburbs mix with sleet or rain. However, precipitation may be so light during this time, it may not appreciably bring down totals. 

Then, during the day on Monday, as the low pressure relocates off the coast, potentially significant bands of snowfall will likely form in or near our area to the northwest of the low pressure center. These bands WILL occur, but the exact orientation and geographical location of them is uncertain. Areas that get under sustained bands of snow will add significant snowfall to the totals from Sunday. Areas that lie between snow bands in areas of subsidence (sinking air) may receive lower totals. 

The early evening run (18z) of the GFS model showing a possible progression of the system. Note that the GFS often does not resolve temperature profiles well during setups like this. Thus, the mixing/changeover to rain may not be as pronounced as shown. (Source: PivotalWeather)

The snowfall from the Monday portion could add double (or more) the totals from the Sunday portion. Ultimately, this should become more clear as we get closer in time to the start of the storm. 

The early evening model runs have shown a general theme that we are getting a snowstorm. But as mentioned above - the details are scattered around and focused in different parts of the region depending on what model you're looking at. 

Storm totals (from both portions of the storm) are expected to be AT LEAST three or four inches across most of the area. The exceptions will of course be to the south and east of the I-95 corridor as usual. However, totals in isolated locations (that get under heavy bands) could be double-digits. 

Early evening (18z) GFS model run total snowfall for both portions of the storm. Note that the resolution of the GFS model may not 100% capture exact banding locations. (Source: PivotalWeather)

Early evening (18z) RGEM (a regional Canadian model) snow totals. Note that this model is higher resolution and can show areas where banding may setup more exact than the GFS model above. However, the banding may not end up being in those spots at all. (Source: PivotalWeather)

I'll be posting a more refined snow map tomorrow morning with some additional details once we have a few more cycles of the models to analyze and digest.

Essentially, you can see above that while the general idea of a significant snowstorm is seemingly "locked in" - the details are going to be what determines the winners and losers in this system. With more model data, and observations of how the storm develops tomorrow night and Sunday, we'll be able to determine who is going to "jackpot" 

Stay tuned! 

Monday, January 25, 2021

Taste of Winter On Tap

 Snow lovers will be disappointed by how the next two potential winter weather events have trended in the models. What appeared to be a week where winter might have roared back with a vengeance, has trended to a much more tame situation. We'll go in-depth below. 

Late in the work week last week, it appeared that a minor to moderate winter weather event would impact the area tonight into tomorrow. Additionally, it seemed that there was significant potential for a larger storm later in the week. However, due to some complex factors in the mid and upper levels of the atmosphere, both events have trended downwards. 

With regards to the winter weather event tonight and tomorrow - temperatures are quite marginal. While favored areas to the north and west (think the I-81 corridor) may see some icing and snowfall, the overall theme of this storm system is minor. There may be a a slushy inch or two even close to the metro area, along with a little dose of icing as well. However, I would think that by tomorrow afternoon, most areas will have safe travel close to the cities. This will be more of a nuisance storm than anything. 

High resolution NAM model showing potential snowfall primarily along and north of the Potomac River. This may even be a bit on the generous side. A slushy inch or two is certainly possible, however. (Source: College of Dupage)

You can see above that the high resolution NAM model shows a little snow accumulation focused north of the Potomac River. Even so, that accumulation should be relatively minor and slushy. 

Freezing rain forecast from the high resolution NAM model. A little icing is possible, especially to the west in the favored colder areas. (Source: College of Dupage)

Icing may be a story as well. Of course, even the tiniest amount of freezing rain accumulation can cause mayhem on the roads. The good news is that temperatures aren't frigid for this storm. Icing is still a concern, but as soon as temperatures rise above the freezing mark, things should improve. Perhaps the I-81 corridor will see a more prolonged period of icing conditions. Regardless, use caution on the roads during the precipitation, and be careful particularly on the bridges and overpasses. 

The attention will then turn to later this week. What appeared to be our first chance of a big snowstorm has petered out. Instead, a storm system looks likely to pass to the south. We may see some flurries around, but at the moment, the chances for substantial snow are going way down. Snow lovers will be stuck waiting again. 

The 06z GFS model run from Monday morning. You can see a low pressure area and its associated precipitation well off the coast. This storm system was forecast to be much closer to the coast a few days ago. (Source: TropicalTidbits)

There are indications that the overall Northern Hemispheric pattern may remain semi favorable for potential snow events going forward, but the problem so far has been a lack of cold air across the entire continent. We'll see where February takes us! 

Saturday, January 23, 2021

Winter May Make a Comeback

With the exception of a few flurries and the snow/mix event back in December, winter (other than some seasonably cold days and nights) has been largely absent this year. There have been multiple factors (and some bad luck) that have contributed to this. 

Technical Discussion

During the fall, the configuration of the upper level weather patterns over the Pacific Ocean caused warm air to essentially flood portions of North America. Our cold air "source" regions in Canada became essentially non-supportive to winter weather events. Even with storm systems tracking favorably for our region, there was simply no cold air to tap or drain into the area.

If you're a regular reader of weather blogs or discussions, you'll likely know that our best snowstorms tend to occur when certain atmospheric teleconnections are in the right configuration. Primarily, we look at the North Atlantic Oscillation (NAO), Arctic Oscillation (AO), and the Pacific North American oscillation (PNA). In an ideal world for snow, we'd have a negative NAO, negative AO and a positive PNA. It's also important to remember that we are in a La Nina pattern this winter, which is generally not as favorable for snow as a weak to moderate El Nino. 

Interestingly enough, we've had those oscillations in our favored spots for large sections of the winter. The problem is, the cold air source regions are just not that cold. This is improving, and this is part of the reason why we now have some winter weather threats to track.

End of Technical Discussion

The first threat for winter weather will be from a storm system that will impact the area from Monday into Tuesday. As usual, the models will shift around until we are closer to the event start time. This particular system was "slipping away" as it appeared that it could be a very light event or a rain event (or both!). However, there has been some tendency in the past few model run cycles to bring the event back to a point where we could see some wintry precipitation. 

The last 12 runs of the GFS model for the Mon/Tue winter weather potential. (Source: TropicalTidbits)

You can see in the above animation that the winter precipitation threat has bounced around a fair amount. Areas that are typically favored (north and west of the metro area and into the I-81 corridor) could see the best shot at winter weather. The unfortunate thing is that those areas may also see a significant icing event caused by freezing rain. Freezing rain is a tricky precipitation type (ptype) to forecast, so this won't be clear until closer to the event. 

The Saturday, January 23 06z GFS model run showing potential freezing rain accumulation. (Source: College of Dupage)

The image above shows the 06z GFS run (the early Saturday morning model cycle) freezing rain accumulation. It shows very damaging amounts of icing for parts of the higher elevations to the west. Even some icing shows up closer in to the DC/Baltimore areas. 

The Saturday, January 23 06z GFS snowfall forecast. (Source: College of Dupage)

Meanwhile, you can see snow accumulations look A LOT less impressive based on this run of the GFS. Of course, this is all still a fluid forecast and a lot can still change. For now, if you are WELL to the west of the metro area, it wouldn't hurt to prepare for some icing. Any forecast for destructive levels of icing should hold off for now until certainty is higher with the storm system. 

Perhaps the more interesting aspect of the forecast is later in the week. Some model solutions suggest a another storm system will pass by to the west before transferring to the coast. This would be on or around Thursday. While the latest runs of the GFS model are showing a significant snowstorm for the region, other models are not yet biting on the event. Thus, we'll leave it as a note here - let's get through the Mon/Tue storm before we look too far into the next one. 

Stay tuned!