JANUARY 2018 - Seasonal WX Week/Pacific Front Saturday

[Andrew]

NAM is showing some good accumulations over the northern section of SE Texas. Similar to last month's event. I suspect we will see winter storm warnings issued for part of the CWA later today. Especially if globals catch on.

[srainhoutx]

Latest surface chart suggest the Arctic Front has passed Oklahoma City and it racing S at a good clip (20 to 30 MPH). Snow is falling a little further N of the front across Kansas and Eastern Colorado. The Arctic High Pressure is rather strong at 1052 about to enter North Dakota from Southern Canada. The shortwave (low pressure) is progressing toward the Great Lakes. The Western US water vapor imagery suggest a weak upper low over the Baja moving East which raise a bit of an eyebrow since the lack of any lifting mechanism other the upglide from the Gulf has been the idea for generating precipitation. If upglide or lifting the atmosphere is the only mechanism generating precipitation, it would tend to be very light (drizzle/mist) versus slightly heavier precipitation with some sort of additional lift mechanism. The current QPF (Quantitative Precipitation Forecast through 12Z Wednesday Morning is only about a 10th of an inch with lighter amounts East of Houston. The highest totals are across NE Texas and to our SW and West. Within the hour we will begin to get the new Mesoscale Model data and hopefully a better idea of what our sensible weather will be to some extent. Remember this is as it always has been in our part of the World a challenging and difficult forecasting scenario, so expect changes as we get closer to tomorrow morning. All that said the overall guidance has trended much colder than anticipated so plan accordingly for a hard freeze possibly colder than the last event this month and extreme wind chill or 'feels like' temperatures.

[srainhoutx]

Monday morning briefing from Jeff:

**Winter Weather Advisory issued for all of SE TX for Tuesday***

Winter Storm Warnings issued N and W of SE TX for Tuesday

A Winter Weather Advisory has been issued for all of SE TX for Tuesday for the accumulation of ice resulting in dangerous driving conditions. The advisory is in effect from 300am Tuesday to 600am Wednesday.

Hazardous driving conditions are likely to develop across SE TX on Tuesday from NW to SE including the Houston metro area.

It is important to understand that unlike most of our winter weather events across SE TX which usually occur during the overnight and early morning hours, that this event will begin near sunrise and worsen during the day into Tuesday night with travel becoming increasingly impacted throughout the duration of Tuesday.

Discussion
Powerful arctic cold front is moving through the plains and will enter TX later this morning and blast off the coast early Tuesday morning. Temperatures will fall rapidly in the post frontal cold air advection with the freezing line reaching a College Station to LaGrange line before sunrise and to Hwy 105 by sunrise and then to US 59 by midday/early afternoon and the coast by early evening. Expect temperatures to fall into the 20’s across much of the area by late afternoon/early evening on Tuesday. Moisture will be lifted up and over the surging cold pool resulting in lift and scattered light precipitation across the post frontal air mass. Strong winds of 15-25mph will drive wind chills well into the 10’s and 20’s on Tuesday and Tuesday night. Widespread freeze with low temperatures well into the 20’s for all areas and possibly upper 10’s N of HWY 105 for Wednesday morning.

P-type:
Will see rain develop tonight ahead of the arctic boundary and then transition to freezing rain early Tuesday morning from LaGrange to College Station to Huntsville. Freezing rain will move southward Tuesday morning likely reaching HWY 105 just after sunrise and then to US 59 by midday and the coast by late afternoon/early evening. Colder air will filter into the region throughout the day allowing some sleet or even snow to mix with the freezing rain especially across the north and western counties where the surface freezing layer will be deepest and the mid level warm layer smallest. Overall the dominant P-type appears to be freezing rain for much of Tuesday morning and then a freezing rain/sleet mixture across much of the area Tuesday afternoon and evening.

Accumulations:
Overall accumulations will be light, but even a trace of freezing rain can be extremely hazardous on bridges and overpasses. Given ground temperatures are already cold from the recent cold weather and temperatures on Tuesday will be falling into the upper 20’s over a large portion of the area…ice formation is likely on all elevated surfaces. As temperatures fall into the mid 20’s Tuesday evening ice may even begin to form on surface streets if they remain wet. Accumulations of ice will average anywhere from .05 of an inch to .10 of an inch across the region.

Impacts:
Expect freezing rain and ice formation to begin before sunrise on Tuesday NW of LaGrange to Huntsville with travel conditions deteriorating as ice forms on bridges and overpasses. Ice accumulation will move southward during the day and likely begin to impact the metro Houston area by midday into the afternoon hours with bridges and overpasses becoming slick. Main concerns will be all bridges and overpasses as accumulations are not expected to be heavy enough to result in downed tree limbs or power lines.

Confidence:
While confidence is high that much of the area will see freezing/frozen precipitation the exact timing of onset and accumulations may change over the next 24 hours. Should accumulations need to be increased a Winter Storm Warning would be required for the area or portions of the area.

Decision Support:

· NW of LaGrange to Huntsville: Onset freezing rain 300-400am Tuesday. Ice accumulation up to .10 of an inch. Hazardous travel all day



· N of HWY 105: Onset freezing rain 600-900am Tuesday. Ice accumulation of .06 to .10 of an inch. Hazardous travel after mid morning Tuesday



· N of US 59 including metro Houston: Onset freezing rain 1100am-200pm. Ice Accumulation .05 to .08 of an inch. Hazardous travel starting around noon/early afternoon



· Coastal areas: Onset of freezing rain 300-600pm. Ice accumulation .01 to .05 of an inch. Hazardous travel starting mid afternoon



· Precipitation should gradually end from N to S Tuesday evening, but very cold temperature will maintain icy conditions through mid morning on Wednesday.

Key Messages:

· Winter Precipitation now likely across SE TX on Tuesday

· Ice accumulations of .01 to .10 of an inch expected over much of the region

· Travel will become increasingly hazardous during the day Tuesday and continue into Tuesday night and Wednesday morning

· Temperatures will fall into the 20’s across the entire area Tuesday night with wind chills into the 10’s (Hard freeze warning may be required for some areas)

· Protection of plant, pipes, pets, and people should be completed today

[don]

The Texas Tech WRF even shows pockets of moderate precipitation across Southeast Texas.

[Andrew]

Gfs is coming in colder and more abundant with precipitation. Still has a large warm nose but seems to erode it rather quickly

[stormlover]

Gfs isn’t bad !!

[redneckweather]

Wxman will be in there shortly to squash y'alls hopes of any wintry precip.

[don]

12z RGEM is aggressive also, i think the reason why the models are getting more aggressive is because they try to develop a weak coastal trough.

[stormlover]

Yep sure is !!

[TexasMetBlake]

12z GFS is definitely more aggressive with precip over southeast Texas in a sub-freezing airmass. I agree with Andrew in that it appears the warm nose, albeit significant, erodes a bit quicker than previous runs.

[stormlover]

Trend is our friend

[WeatherDuck]

What sort of total precipitation is the RGEM showing for the Austin area?

[MontgomeryCoWx]

[srainhoutx]

NWS Brownsville issues Winter Weather Advisory

[stormlover]

Cmc colder with more precip

[don]

Yep the CMC is finally on board! its been one of the drier models for the last few days.

[MontgomeryCoWx]

I just went through all of the snowfall maps (GFS, CMC, Euro, NAM, RGEM, WRF). It paints a picture of precip in everyone, with the low end being a dusting and the high end being around 3 inches in the Brazos Valley down to Montgomery County. Houston's average is around 1 inch but I'm sure that is including frz rain and sleet.

[TexasMetBlake]

Looks like we may see an upgrade to a Winter Storm Warning this afternoon...

All joking aside, arctic front is on its way located from the
Ozarks down through central Oklahoma into the Texas Panhandle.
High pressure of ~1052mb over Montana will be pushing into the
northern Plains behind the front by Tuesday morning. Broad
positive tilt trough continues to develop on water vapor imagery
with upper low over the Great Lakes and vorticity max/jet streak
coming down through the Plains. Front should push through the area
late tonight into Tuesday morning. We are still monitoring new
model data for winter precipitation development. The 12Z NAM and
GFS seem to be hinting at freezing temperatures occurring sooner
in the day. We are not sure how this may impact our current winter
weather advisory or how this affects precipitation types, but
there will be some adjustments for the afternoon forecast. Early
analysis shows quite a bit of frontogenesis through 925-850mb and
850-700mb layers so mesoscale precip banding is a growing concern.
The real trick is timing these precip bands versus the intrusion
of dry air from the north through the boundary layer. This may
determine when precip may end on Tuesday.

[davidiowx]

I just went through all of the snowfall maps (GFS, CMC, Euro, NAM, RGEM, WRF). It paints a picture of precip in everyone, with the low end being a dusting and the high end being around 3 inches in the Brazos Valley down to Montgomery County. Houston's average is around 1 inch but I'm sure that is including frz rain and sleet.

An inch of freezing rain would be a nightmare here!!

[srainhoutx]

Just released Update from Jeff:

After reviewing the latest meso scale guidance some changes are required to the onset timing of freezing precipitation on Tuesday. This timing will also match better with current NWS timing graphics on their webpage.

Short range guidance is showing a faster and more aggressive frontal passage over the area late tonight into early Tuesday with a faster progression of the freezing line across the region and to the coast. It is increasingly likely that the freezing line will reach the US 59 corridor by 800-900am and toward the coast west of Freeport by noon. This will result in an earlier onset timing of the freezing precipitation over the region and deteroriation of travel conditions especially on bridges and overpasses.

No changes are currently required to accumulations across the region. In fact some of the short range guidance rapidly ends precipitation from N to S Tuesday afternoon while the larger global models hang on to the precipitation a little longer into the evening hours.

The following timelines have been updated to reflect the latest thinking


•NW of LaGrange to Huntsville: Onset freezing rain 200-400am Tuesday. Ice accumulation up to .10 of an inch. Hazardous travel all day


•N of HWY 105: Onset freezing rain 400-700am Tuesday. Ice accumulation of .06 to .10 of an inch. Hazardous travel early Tuesday morning


•N of US 59 including metro Houston: Onset freezing rain 800-1000am. Ice Accumulation .05 to .08 of an inch. Hazardous travel starting mid morning


•Coastal areas: Onset of freezing rain 1100am-100pm. Ice accumulation .01 to .05 of an inch. Hazardous travel starting around noon


•Precipitation should gradually end from N to S Tuesday evening, but very cold temperature will maintain icy conditions through mid morning on Wednesday.