2/13 Rain to Snow Event for Morning Commute

This will be a quicker post since I only have a little time today. But we went from a generally all-rain storm to potentially quick heavy snow in less than 24 hours. The South trends on modeling with this short of a lead time are quite remarkable, and I'm not sure if I've seen anything like this, at least recently.

 A dynamic system will be approaching from the TN valley, bringing precipitation to the mid-Atlantic and Northeast, a lot of which will be snow. Winter weather advisories are up for those NW of 95 and N of I66. 

We will start as rain, but likely switch over to snow and see accumulations in these areas. For us, we get cold enough to snow from dynamic cooling; this system will be very dynamic. It depends on rates, and the boom/bust scenarios are equally likely. The bust scenario would be that we do not get heavy rates and little to no accumulation; the boom is that we get heavy rates for a few hours. Despite marginal temps(33-34), heavy snow rates will accumulate, even on sidewalks/roads if heavy enough. This storm will be more of a latitude and elevation-based storm rather than longitude. That's why central/northern MD is more favored in this setup. Even with heavy rates, your ratios will be low and it will be a wet snow.

Precip will likely onset between 5-8 AM, with central MD expected to be all snow by 7-8 AM. Guidance is indicating heavy precip rates, which timed with the morning commute will make it hazardous. It's not more so about the actual amounts, but the heavy rates expected and the timing are perfect to impact the commute. This storm has trended so far south that Maryland gets in on the main snow shield; and not just the backend. Our storm's preformance will depend on these rates. 

There is the potential for 1-3 hours of 1-2"+ rates during the morning commute. With these rates, it will accumulate, even on side roads and paved surfaces. Heavy rates always beat marginal temperatures or any other factor. The previous rain will also make pretreatment impossible. The heaviest rates will likely be just after the switchover(when dynamics are the most favorable), with thundersnow possibilities being hinted by guidance. We won't know what this storm will do until we get there; it is fully dependent on the level and location of banding.

Snow Map + School Predictions:

HREF forecast for the next 24 hours; 2-4" NW of 95 and N of 66 with 4-6" for 70 and north

This is my snow forecast for the event; I think there should generally be at least a solid inch for N of 66; further south should stay under an inch. 3-6" for elevated areas near the MD line and of course, the mountains look like a good call. especially with the recent trends. I would not be surprised at all of the warnings were issued last minute for those regions. The plus in the 1-3 zone is mainly for higher elevated areas. This is a very difficult forecast, with an equal chance for a boom and bust. 1-3" is just in the middle zone and I feel like it will either be a bust/boom. Either way, the morning commute is looking awful, and there will be impacts. As for school impacts...

Prediction: 3 Snowflakes
I'd expect many school impacts, with widespread delays and even closures, especially for counties in the 3-6" zone. The timing of this is perfect to at the very least delay. There is also a 50/50 chance for a closure depending on how long the rates last, and how intense they get. If the rates last for long enough, schools will be forced to close, and there will be enough accumulation on the roads. With the current forecast in place. and the rates guidance is showing,  there is really no way schools can open on time tomorrow.

Overall, a nice sneaky event. I think this is going to surprise a lot of people; enjoy the snow!

1/19 Light-Moderate Snow Event

Synoptic Setup 

A vigorous shortwave/trough will move through the area around tomorrow at midday, providing strong lift to generate snow and sufficient dynamics from the strong upper jet, in addition to PVA and modest WAA. These dynamics will allow heavy precip to be brought about just ahead of the shortwave and a more expansive precip shield, which guidance has been trending to. Guidance, in addition to the more expansive precip shield, has brought the vort-max further south, allowing for higher amounts of QPF. The heaviest snow/concentration of precip will just be north of the vort-max; putting us in a more favorable position. 

This is ahead of a great antecedent airmass, with already existing snow cover contributing to it even more. However, the absence of deep moisture, and generally low precip amounts, as well as the general storm being too progressive will inhibit this from becoming a major storm - despite the favorable H500 pattern. Nonetheless - a moderate storm for the area is looking likely, with ample cold. Ratios will be high due to a cold, saturated profile in the DGZ and the rest of the column, leading to modest dendrite growth. Surface temps will also be in the upper 20s and lower 30s the entire event; below freezing the entire time. 

We should see a good thump of snow not too far from onset; making morning commute extremely hazardous. Shortwave forcing by that time is very good and would support locally heavy rates; despite low lift in the DGZ. This will likely be another high-ratio, generally low-rates event. Roads will become slick and snow-covered extremely quickly, with lingering snow expected to last through the evening rush hour. Thus, most schools/offices have closed in person. Snow will continue throughout the rest of the day, but will become lighter after the morning thump. 

Snow Map

This is my forecast:

I went extremely bullish due to the uptrends today and the overall setup. The most snow will be N of I70 in NE MD due to great dynamics and precip from the IVT(which won't have that much of an impact closer to DC). N of I-66 should generally see 2-4"+ (why I brought my 3-5"+ zone further south), with the south of it generally seeing 1-3". Although this is bullish -  please keep in mind that just cause a specific area is in the 3-5"+ zone doesn't mean they will be seeing the higher amounts of the range. Most areas S of 70 in that zone will likely stay under 5". The further north and east you are, the better for this setup(outside of the mountains). The exact location of the bands and the heaviest QPF/snow are uncertain, thus why I added the plus for any isolated spots above the range. The general consensus should be another nice high-ratio, low-rate snow event - on top of already existing snow, something that is quite rare in the mid-Atlantic. This storm will have a large impact on the morning commute and bring about hazardous travel.

Additionally, radar looks good right now, with virga saturating the column currently, as well as a decent precip shield in the OH Valley/Midwest. MPING reports confirm this is all reaching the ground over there. Earlier today, HRRR underdid the moisture in IA and didn't see the snow shield reaching Cincinnati (all related to our storm system). Current observations look good so far and aren't indicative of any major concerns.

Overall, the last-minute uptrend on the vort-max being further south and a more expansive, robust precip shield have led to totals becoming higher in the DMV.

Here are the current NWS warnings/advisories map, and precip onset:

12/10/2023 Accumulating Snow Event NW of I95

 Guidance has converged on an anafrontal system with a secondary low that provides the opportunity to see some snow, as moisture is thrown from the secondary low into the cold sector. Thus, this gives us to potential to see snow falling, with (light) accumulation prospects NW of 95.

SYNOPSIS:

A shortwave, which has already moved through the country and continues to do so has a cold front ahead of it and northern piece energy. The trough axis is delayed behind the front, so we set up an anafrontal event(not typical).

 This system overall is very dynamic and should have good PVA in front of it which induces cyclogenesis(intensification of the storm) and moisture advection. Temp gradient is also quite large, with the airmass behind the cold front being Canadian and the airmass in front of it being subtropical.

We also have a dual jet setup associated with this which pops a secondary low when the trough becomes negatively tilted, in the Carolinas. This directs winds and moisture return NW.

The large temp gradient should allow for frontogensis forcing, which will make precip rates extremely high. CAA driven snow will occur in the overnight hours after midnight for most people as temps drop.

One thing to note about this system is that surface temps are extremely marginal, but the rest of the atmosphere is <0C, allowing for it to snow. These heavy precip rates will allow for greater dynamic cooling and a saturated column.

    

OBSERVATIONS:
We are now less than 12 hours away from snow, and the use of models is useless. As of 23z(6P EST), the trough is netural-slightly negatively tilted over MS/AL.

This is ahead of schedule, as it was supposed to be this way at 7-8 PM. This will have better implications downstream as a earlier negative tilt will allow for cold air to fill in faster during the snow, and the secondary low to pop more/quicker in a favorable location.

The cold front was also ahead of schedule, and stronger than modeled, passing the BR by 515-530 PM. 

Temps are dropping quite quickly behind the front, with some station reporting degree drops every 4-5 minutes. Although it is worth noting that sfc temps are warmer than on NAM, they are colder than on HRRR(behind the front), and it just initalized at 22z

The rate temps are dropping behind the CF, catching up to NAMs temps wouldnt take too long either. It's running too warm for some parts of nova at 0z anyways. We'll see though, eitherway temp wise we look good.

And now we look at the radar, we are starting to see the backside precip fill in and amplify. It looks very good with convection off the Carolinas. This system is absolutely juiced, and is exceeding expectatations. 

So overall, it might be safe to say that this event is going slightly better than expectations. Observations look great so far and is expected to continue into the event. 

Overall:

Overall, despite uptrends, the general picture is the same: accumulating snow for NW of 95, mainly in higher elevations. Temps will be very marginal(32-36), but we might push colder during heavy bands due to CAA. That being said, we need heavy snow rates to stick. Otherwise, it will not stick; moderate or below will not do it. Mesoscale banding features are too hard to resolve at this range, so there is no really saying where it will be, but if I had to guess it would be somewhere around here(NW of the fall line): 

As for snowfall, there will be an extremely sharp cutoff on I95. I have trouble thinking anyone inside or east of the beltway will see more than just white rain, with little to no accumulation. Elevation is just too bad there to get anything substantial. Temps will be too marginal there for anything. Even further NW, temps will remain 32-36 for most of the storm, with the exception of lower temps 1000'+ or in the bullseye areas. The reason I'm even going this bullish on accumulations is because of the puking snow rates that will set up somewhere that will stick to the ground despite temps. The rates cannot be moderate or light, they have to be heavy, but we know it will set up somewhere,, and that area should see nice accumulations. Having heavy wind should also help blow the snow around and even help with stickage as wind removes latent heat release; it would help us more than hurt us. This, along w/ heavy rates will make for low visibillity conditions. 

As of N of the fall line, I gave the highest amounts to areas 700'+ elevation and west of I81. The bullseye for this storm overall will be the Blue Ridge and the Apps, as expected. Locally, the bullseye is probably the Parrs Ridge if I'm betting.  In the Parrs ridge, there could be a spot of over 3"+ if everything goes right. I went extra bullish based on new data we have, but this could very well bust. I feel like this is a boom/bust type setup; either we do really well with banding and get puking snow for hours accumulating to over an inch, or we generally stay mostly rain with no acccumulation even if it snows. Ratios will be very low; likely 4:1-7:1 depending on your location.

And as for school impacts, models have trended earlier with the event, generally wrapping snow 5-7A through the region, but before that it will be absolutely puking, along with strong winds as the low intensifies and moves NE. The snow will melt extremely fast afterwards, but I still think theres a chance we could pull off a delay. I'll go 2 snowflakes(50% delay) for this event, if band is a bit later then we'll probably delay. If we can get a WWA in place we probably delay, but it can go either way. Accumulations will mostly be on grassy surfaces, but depending on rates, could be on some side roads and sidewalks. Overall, this event should lock in Dec measurable snow for most areas, and should be really nice to see. 

March 12, 2022 Snow: Strong Winds and Near-Blizzard Conditions

Good evening everyone! Over the past week or so I have been tracking a winter storm threat for the DC area on Saturday. In the past 24 hours, models have consistently trended colder and more southeast, allowing for our region to garner higher snow accumulations. There is a winter weather advisory for I-95 corridor and north and west of that. 2-4" is expected in I-95 corridor, while 3-5" is expected NW of 95. The cold air will come in quickly and strong tomorrow. There will be a brief period of sleet while we transition over to snow tomorrow, which will help snow accumulate better. The freeze will also aid with this. The winds will be very strong tomorrow. Many areas will see wind gusts in excess of 50 MPH, up to 55 MPH. Due to winds and the heavy snow expected, blizzard conditions are possible tomorrow,  particularly between 8 - 11 AM. In the WWA, it states the possibility for this. Thundersnow is also possible, mainly when we transition into snow(soundings support it). Winds are quite undermodeled. Tomorrow morning, we will have an FGEN band that will setup west of DC, but east of I81 as the transition to snow occurs. Rates could be 1-3" per hour in this band. Because of this, I really think west of DC should be placed under a winter storm warning. Just because it was 60 today does not mean snow will not accumulate. There have been many times where there were mild temps before onset of snow, and areas got a good amount of snow(1/3 as an example). I'll show some of the 18z mesoscale models here.

 

HRRR: 

 

The HRRR has a very impressive snow accumulation, with the bulk of the snow falling in 6 hours. Latest runs of the HRRR are uptrending on snow accumulation too. I think this could be a legit overpreformer for our region, dynamics and FGEN banding are very impressive. The soundings from the HRRR scream FGEN band(winds rapidly veering with height, ample amount of omega and lift in DGZ) with good snow growth. I would cut totals by about 1/3 for a more realistic amount, but still a very impressive amount.

NAM 12KM:

NAM is quite impressive with snow accumulations, with a widespread swath of 6" and higher. Again I would cut totals by 1/3 - 1/2, but this was a big improvement from last run. The reason why is that NAM is slower, and colder and east, allowing for more snow accumulation. NAM 3KM supports this too.

NAM NEST(3KM): 

The NAM 3KM is very impressive with dynamics + FGEN and supports the low-resolution NAM's solution. NAM 3KM is also impressive with a wide swath of 6"+. Again cut totals by 1/3-1/2. Much better than last run.

NAM 3KM has an impressive FGEN band moving through the region as rain turns into snow.  A sounding reveals ptype being snow, with good dendritic growth. Winds rapidly veer, which indicates FGEN. There is good lift in the DGZ with nice omega values. This is a good sounding for heavy snow, and for strong dynamic cooling. This sounding also indicates some possible thundersnow in some areas. 

My snow map:

Here is my final snowfall map. In the red, you can expect 4-8" of snow(Blue Ridge, Catoctin Mountains, Western MD). North of DC, you can expect 3-6" of snow. Most of the region DC and south are in the 1-3" range. Far southeastern parts of our region can expect C-1". I went a little bit bullish on the snow map because of latest trends on guidance leading to more snow. The dynamics and setup from this event is quite impressive. Even though rates are still uncertain, a FGEN band that will allow for these totals to come to fruition. The north winds will help blow snow and help transport cold air here. When rain transitions over to snow at around 5-8 am for most of the region, there will be a brief period of sleet. This will help snow accumulate, including the flash freeze. This will be a nice event for March. Whiteout conditions at times cannot be ruled out, and near blizzard conditions for somewhere are likely at one point at time. Even thundersnow is possible tomorrow(transition to rain - snow). This will probably be the last snow event looking at long range models, so enjoy everyone! Main roads should be fine but secondary roads will likely cave to the high snowfall rates.

Wintry Mix Thursday - Friday

Good evening everyone! We have a threat for a wintry mix from Thursday to Friday, mainly north of DC metro. Models have been trending south and colder for the past day or two. The highest ice will be near the mason dixon line. Unlike the other ice event that busted, this is a cold air damming event, which tends to be undermodeled. This will not be a snow threat, and the main focus of this will be ice and sleet. Although precipitation onset could be snow, almost all of it will turn into sleet/ice quickly. Precipitation onset has been delayed a few hours to after sunset, which would lead to a better environment for ice to accumulate. Main roads most likely won't be a problem, however secondary roads and sidewalks could have some slick spots, especially north of I-70 and in higher elevation areas such as Damascus. A potential delay is possible on Friday, but I won't make a prediction until we get into the event and surface obs.

Now, I will show what some model simulations are for this storm. 

 

RGEM/RDPS:

This is the RGEM model. It has done quite well this winter. The location of the ice accumulation is realistic, and fits with climatology. However, ice totals are still definitely overdone. The RGEM would give us a scenario for a delay, as ZR lasts in Damascus for until 11a-12p. This solution matches up more with surface temperatures. It has shifted south and colder compared to its last run.

 

Euro:

 

Euro is a bit more conservative with the ice totals, but still overdone in some places. It has shifted south and colder compared to its previous run. For Friday 1a, it gives Germantown rain on ptype map, when it should be ZR as temperatures are 32, supporting freezing rain. The Euro and RGEM line more up with my thinking and climatology on where ice accumulations will be.

About the NAM. It has a cold bias and spits out a very cold solution, with ice accumulation for even southern parts of our region. Although it does do well with CAD, it is quite unreasonable. Ground temps will not cool instantly, and NAM assumes it does. As I said earlier, it has a cold bias, so it is also unreasonable in the ptype solution it puts out. Even with later onset time, I just don't see it happening. 

 

Here is my first call ice map:

 

I will have an update with a school prediction for Friday and with mesoscale models + surface obs tomorrow.