JANUARY 2018 - Seasonal WX Week/Pacific Front Saturday

[srainhoutx]

Area Forecast Discussion
National Weather Service Houston/Galveston TX
338 PM CST Sat Jan 13 2018

.DISCUSSION...

...Today through Tuesday Night...
Quiet and cool weather this weekend will gradually warm to the
upper 50s to mid 60s by Monday as high pressure slides east and
onshore flow returns. Low temperatures tonight are forecast to dip
below freezing across most of the inland areas, but a hard freeze
is not anticipated.

Rain chances increase Monday evening through Tuesday as a cold
front pushes through the area. The predominant precipitation type
is expected to be rain, however there is still a slight chance of
some wintry precipitation mixing in across the northern parts of
the area early Tuesday morning. At this point it's still a little
to early to nail down precipitation type, and for now all types
of frozen precipitation (snow, sleet, and freezing rain) are on
the table. It really comes down to how fast the cold air sweeps in
behind the front vs how quickly the post-frontal precip ends. The
ECMWF is faster with the front bringing it through more overnight
Monday night, vs the GFS/NAM solutions of a Tuesday morning
frontal passage. Previous ECMWF run showed faster drying behind
the front in addition to being quicker than the others, however,
the latest run is showing some lingering precipitation well after
the freezing line passes. Although confidence is slowly increasing
that frozen precipitation is possible, overall chances are still
fairly low. Right now, the best chance for frozen precipitation is
along and north of a Columbus-Conroe-Livingston line. If frozen
precip falls, light accumulations (<0.1 inch) will be possible.
With temperatures falling throughout the day Tuesday and into
Tuesday night, any wet surfaces remaining will likely freeze with
low temperatures in the low 20s to low 30s Tuesday night. A Hard
Freeze Warning may be required for portions of the area. 11


...Wednesday through Saturday...
High pressure builds in Wednesday behind the cold front and will
quickly progress eastward across the Central Plains. Onshore flow
and moisture will return Thursday as the surface high moves over
the Tennessee Valley. An upper-level shortwave trough moves
through the area Thursday night with strong warm air advection and
lift, so precipitation is expected Thursday night through Friday.
The biggest issue is if the precipitation begins early enough on
Thursday to catch the below freezing temperatures, freezing rain
is possible along the northernmost counties. Confidence is
currently low enough that no fzra was included in the grids, but
it is something we will watch out for moving forward. Behind the
shortwave, a gradual warming trend will continue through the
weekend before the next cold front passes on Sunday. 22

[Heat Miser]

Looks like Tues-day will be the best chance, unless we have something developing post-frontal which brings a little more moisture for a longer period of time. Something that brings in more moisture Tuesday afternoon into the night.

[DoctorMu]

It looks like a more northern county even for mixed precip on Tuesday. Dry air will be the largest impediment.

[mcheer23]

NAM is more agressive as far as frozen precip.

[srainhoutx]

NAM is more agressive as far as frozen precip.

For a model that typically over estimates QPF and has been extremely dry, it's a noticeable change.

[harp]

The NAM is the one model that nailed our snowfall here in Louisiana last month.

[stormlover]

Gfs is on board more now

[harp]

Gfs is on board more now

Not for SE La.! LOL!

[Andrew]

Interesting to note the delayed shortwave that could sweep through the region on Wednesday/Thursday bringing another chance for wintry precip. I definitely think the chances are there for more than one opportunity for wintry precip across the region.

[don]

Euro is also wetter showing snow in Houston Tuesday night. It also still shows a ice storm on Thursday fwiw

[Andrew]

Euro is also wetter showing snow in Houston Tuesday night. It also still shows a ice storm on Thursday fwiw

Yea ECMWF is showing a good mix of precip across the region. It's looking more and more likely that north of Highway 105 will experience wintery precip. Chances will remain for south of that line but further north I am becoming more confident that a mix of precip and possible travel impacts will occur.

[srainhoutx]

We should get a better idea today into tonight what our sensible weather may bring early Tuesday morning into Wednesday. The Cold Front that will bring this next chance of wintry mischief to Texas is currently crossing the Dakotas heading S behind a strong upper air disturbance moving toward the Great Lakes/Mid West. The overall Western Ridge/Central/Eastern trough configuration continues to hold strong with embedded disturbances riding up and over the Ridge in Alaska and dropping S to SE in the Yukon/NW Territories, The Wednesday night disturbance is currently over the far reaches of NW Canada and will likely take another 24 to 36 hours to resolve in the model schemes.

Monday night into Tuesday night as of now suggests the Hill Country, Austin/San Antonio, La Grange and point S of there have the best chance of seeing freezing rain/sleet with lesser chances for points East of those Regions, but is subject to a lot of changes. For example the 06Z GFS suggests a wintry mix makes it into the Houston Metro area Tuesday afternoon. Expected changes throughout the next 24 hours or so as this is a tricky and difficult forecast to pinpoint.

[srainhoutx]

Sunday morning briefing from Jeff:

Winter Weather threat increasing across TX this week

Mixture of rain/freezing rain/sleet/snow across much of TX Tuesday and possibly again on Thursday

Mixture of rain/freezing rain/sleet potential increasingly across SE TX (N of HWY 105 on Tuesday)

I will start with the emphasis that any winter weather event in this area is low confidence and things will almost certainly change over the next 24-48 hours…residents should remain updated on changes to the forecast and possible impacts to travel both locally and across the state.

Discussion:
Very cold arctic air mass is on the move this morning and will surge southward across TX on Monday. Front will move off the coast late Monday with temperatures falling in the post frontal cold air advection. Freezing line will surge southward early Tuesday and likely reach our northern counties (College Station to Livingston) around sunrise on Tuesday. Freeze line may continue to build southward into the morning hours on Tuesday approaching the HWY 105 corridor while most of the rest of the region falls into the mid and upper 30’s under strong N winds. An upper level disturbance will approach and move across the region in the post frontal air mass on Tuesday and Tuesday evening. This feature will induce southerly winds over top of the surface cold dome result in an overrunning situation and the likely formation of precipitation across the region. Given the deepening cold air across the region nearly every P-type may be in play initially as rain and then changing to freezing rain (HWY 105 northward) Tuesday morning and then mixing with sleet and possibly snow during the afternoon hours as the air column cools. South of HWY 105 surface temperatures may remain a few degrees above freezing which would prevent freezing rain, but as the air column cools the rain may mix with sleet/snow. South of I-10 surface temperatures will likely remain well above freezing resulting in mostly rain with maybe some mixed in sleet.

It should be clearly noted that much of the forecast for Tuesday greatly hinges on surface temperatures and how far south the freezing line progresses as this will determine where any freezing rain and ice accumulation may occur. It is important that residents understand the uncertainty in a 48 hour temperature forecast and that only 1-2 degrees could make the difference P-type over a particular location and potential ice accumulations on bridges and overpasses.

P-type:
Forecast soundings show a warm layer of air above the surface cold dome on Tuesday which supports a rain or freezing rain setup. For freezing rain the surface temperatures must fall to 32 or below so it becomes critical where the freezing line establishes. Freezing rain is liquid water that falls into a shallow sub-freezing layer near the surface and freezes into ice on contact with “freezing” objects such as tree limbs, bridges, roofs, power lines, vehicles. During the day the surface cold layer will deepen and this may allow a mixture of rain/freezing rain/sleet across portions of the area…mainly N of HWY 105. Think snow is least likely given that the warm nose aloft only slowly erodes and will favor sleet over snow in this setup. Won’t rule out snow Tuesday afternoon and evening if the moisture lingers long enough for the air column to cool…think this is most likely N of HWY 105.

Accumulations:
Appears some degree of accumulation of ice will be possible on Tuesday especially in the corridor from Lake Somerville to Huntsville where surface temperatures will likely be below freezing much of the day. South of this line any ice accumulation will depend on the surface temperature falling to 32 or below. Travel across central TX into the W/NW portions of SE TX may become hazardous on Tuesday and into Tuesday night. Should have a better idea of any ice accumulation amounts later today into Monday.

Tuesday Night:
Very cold arctic air mass will entrench across the area with most locations falling into the 20’s. Disturbance will begin to pull E of the region allowing drying northerly winds to shut off precipitation. Upglide does continue into the evening hours around Matagorda Bay and may see the rain change over to freezing rain as the surface temperatures fall below freezing with some ice accumulation possible around the Victoria area NNE toward Colorado and Wharton Counties especially on bridges and overpasses. Will need to monitor how quickly the dry air advects into the region as this could either end or extend the precipitation over the region on Tuesday evening.

Additionally, any remaining residual water on bridges and overpasses may freeze. Will need to watch the southern progression of the freeze line Tuesday afternoon and evening for freezing of any leftover water which could remain in place into Wednesday morning.

Thursday:
Cold arctic high pressure cell will begin to slowly retreat toward the east, but another disturbance will approach the region late Wednesday into early Thursday. Appears this disturbance could cause another round of mixed precipitation over the region. For now this event is still far out and will not attempt to determine any sort of P-type or accumulations…just making everyone aware and to pay attention.

Key Messages:

· Cold arctic front will arrive late Monday with temperatures falling into the 30’s on Tuesday

· A mixture of freezing rain/sleet/snow will be possible from Lake Somerville to College Station to Huntsville on Tuesday with some accumulation possible

· A mixture of rain/freezing rain/sleet/snow will be possible N of HWY 105 Tuesday with little to no accumulation currently expected

· Houston metro area should see mainly rain possibly mixed with sleet Tuesday afternoon…no accumulation currently expected

· Travel to central and north Texas on Tuesday will become hazardous…plan accordingly

· Lows Wednesday morning likely in the 20’s for many areas with wind chills in the teens

Uncertainty:

· Where the surface freezing line is located or how far southward it may move on Tuesday

· How much moisture will linger post front which determines the amount of precipitation and accumulations

· How long precipitation may linger in Tuesday evening as the freezing line progresses southward

[don]

12z GFS is more aggressive. It shows 18 hours of frozen precip in the Austin area, and 12 hours of frozen precip in the Houston area. The Texas tech wrf also looks to be onboard with frozen precip into metro Houston

[stormlover]

German model is interesting

[harp]

German model is interesting

Can you elaborate?

[stormlover]

https://www.tropicaltidbits.com/analysi ... 11412&fh=6

[don]

12z Euro is closer to the GFS showing more frozen precip in Metro Houston and the coastal counties

[WWxHopeful]

About what timeframe would any city officials in the northwestern areas of Harris County most of Montgomery County start to think about issuing warnings, possible school delays or closures, etc. Last thing I want is someone to take this lightly and people to head out to work/school Tuesday and get stuck due to roads being in poor condition.

[srainhoutx]

About what timeframe would any city officials in the northwestern areas of Harris County most of Montgomery County start to think about issuing warnings, possible school delays or closures, etc. Last thing I want is someone to take this lightly and people to head out to work/school Tuesday and get stuck due to roads being in poor condition.

Tomorrow is a Federal Holiday and TXDOT has pretreated some roads/flyovers/overpasses since last Friday. As is usually the case around here with these borderline events, look for tomorrow afternoon to be decision time for exactly where wintry precipitation may impact traveling. I wouldn't be surprised to see a Special Weather Statement issued this afternoon from HGX for our SE Texas Forecast Area.