Browse Tag

18

Winter Storm Threat: February 17-18 (Only/Final Call)

A nail-biter between warming low-to-mid level air and marginal temperatures/wet bulbs at the surface. At least the Sun won’t be fighting most people during most of this event from D.C. northeastward.

Biggest uncertainty lies with snowfall rates, especially in areas that have marginal wet bulb temperatures at or above freezing (i.e. anywhere south of the Frederick, MD to Philadelphia line, generally speaking). That wet bulb freezing line has also been creeping further north than what I had anticipated when looking at the setup over the past few days. We’ll see it the wet bulb temperature or the snow/sleet rates win.

Winter Storm Threat: Feb 18-19 (second call)

Everything trended south in the last 24 hours…

 
So what happened? The main northern stream energy that was supposed to interact/guide the southern energy creating the snow storm has actually moved out ahead of the storm. This changes a few things…

1. The boundary layer will be able to get cooler and drier ahead of the system.

2. The storm itself (the southern stream energy) has slowed a bit and is allowed to stay further south.

3. Less phasing/interaction with the northern stream leads to a weaker, less amplified system.

There will still be some boundary layer issues along the southern line of the snowfall area, but the boundary layer impact on accumulation is significantly less than what we were looking at a day ago.

Many risks still plague this forecast, and they are

1. The storm could be even more suppressed, which would shift the snow further south.

2. Some models still show a more amplified solution, expanding the precipitation northward.

3. Convection in Alabama and Georgia cutting off some of the moisture feed from the Gulf, which would result in lower QPF and lower snow totals.

I will probably make my final snowfall forecast late tomorrow after the 18z models come out. There are still some big differences between the models, but the trends have been hard not to notice. The most likely change would be shifting the whole thing south.

Winter Storm Threat: Feb 18-19 (first call)

What a nightmare of a forecast…

 
If I didn’t already say I was going to put out a snow map today I would probably have waited until tomorrow for a first call. As it is, I might do a second update before my final call on Saturday if the forecast seems to converge well over the next 24 hours.

As has been the case all winter, the boundary layer temperatures are going to be a big issue with this event. Most (if not all) of the snow will be on the back end of the system, which doesn’t help matters, either. The latest models would suggest pulling the QPF further south and maybe some of the snow totals along with it, but it wouldn’t be much further south than what I currently have depicted. With high enough snow rates being depicted by the models on the back edge of the system, I think the snow will be able to stick eventually despite the surface temperatures being in the 33-36 range in the lower elevations. Localized 8-12 inch accumulations will be possible in the mountains that get the higher QPF totals.

I believe that the 4-8 inch snow contour could/should extend east of the mountains at some point, but with the discrepancies between the models at 12z and the inconsistency in the run-to-run storm evolution it would be difficult if not impossible to accurately place it east of the mountains.

Snow flakes in the Mid-Atlantic tomorrow?

Some snow flakes could mix in with the rain tomorrow in the Mid-Atlantic as colder temperatures push into the region. No accumulation is expected for most, but some of the higher elevations in the Appalachians could see a little accumulation.

Surface temperatures are likely going to be too warm for any sort of accumulation in the lower elevations… with most areas in the upper 30s or lower 40s. The freezing level is forecast to be somewhere in the 925-950 mb range, which gives the snow enough time to reach the surface before melting.

Snowfall rates are expected to be fairly light if/when the changeover does occur, which is more evidence that the chance of accumulation in the lower elevations is slim.

EDIT:

Here’s a sounding of the 12z GFS forecast for 1pm tomorrow… the 12z NAM has a lower freezing level than the GFS.


CLICK TO ENLARGE

Image courtesy of Twisterdata.com

The “So Close” Storm of December 2010 (18th-20th)

As this potential snow storm begins to materialize in the Southeast, it’s time to take a serious look at snowfall predictions. This storm appeared to be going out to sea at the start of this week, then for a couple of days it looked like the I-95 corridor would land a rare epic La NiƱa snowstorm along the East Coast. Unfortunately for the snow lovers of the East, the models are converging on a system that heads out to sea (OTS), with a quick snow dump over southern Virginia and a wintry mix in the Carolinas before sweeping northeastwards tantalizingly-close to the coast. The coastal areas of New England also have a shot at some snow.

Most of the precipitation will be off the coast, giving the fish a good time while the snow lovers despair over what could have been. At one point, 6-12 inches along the I-95 corridor seemed like a possibility. Now they’ll be lucky to get a dusting. Northern New England could get some backlash snow from this system after the 20th when the system tries to retrograde back to the west.

Here’s an estimate of what this system will put down on the 18th through the 20th (Sat-Mon):