#2007 Postby EnnisTx » Mon Dec 31, 2018 7:35 pm
Good long read from NWS FW
LONG TERM... /Issued 420 PM CST Mon Dec 31 2018/
/Wednesday Morning through Sunday/
The main concern in the long term period is the potential for
icing across the Big Country and parts of North Texas late
Tuesday into Wednesday. Some of this icing could be significant in
spots. Another bout of wintry weather is possible late Thursday
and into Friday. High temperatures on Wednesday and into Thursday
will be below seasonal norms before conditions rebound to above
normal values for the weekend. The next appreciable chance for
rain will not arrive until late in the weekend and into early
next week.
...Possible Winter Weather Event Rundown....
The latest model guidance suggests that the greatest icing
amounts will be generally near and west of HWY 281 and a Winter
Storm Watch has been issued. This watch goes into effect on 0900
UTC Wednesday (0300 CST) and will run through 00 UTC (1800 CST).
Spatial and temporal changes to the watch are probable. Light
Freezing rain is expected to begin a few hours after midnight on
Wednesday south and west of a Decatur to Glen Rose to Lampasas
line. Through the day on Wednesday, a slow transition from a cold
rain to freezing rain is anticipated to transpire across much of
North Texas and the Big Country and by Wednesday evening locations
west of a Bonham to D/FW to Killeen area will be at risk for
light freezing rain. Areas east of this line will likely remain
too warm for any winter precipitation. On Thursday
afternoon...freezing rain may actually transition back to just a
cold rain briefly for some locations....particularly near the I-35
corridor. A winter mix (perhaps snow/sleet mix) will be possible
near and north of the HWY 380 corridor late Thursday into early Friday.
Temperatures should rise above freezing across much of North
Texas and the Big Country by Thursday afternoon, with some
sunshine possible mainly south of I-20 and west of I-35 before
noon Thursday. Most other areas will likely remain cloudy through
Thursday evening.
...Impacts from Wintry Weather...
The most noteworthy impacts from any winter weather will be
across the Big Country and parts of North Texas...near and
northwest of a Goldthwaite to Stephenville to Gainesville line.
Here, travel conditions may become slick and hazardous due to
icing. Accumulation on trees and powerlines will be possible as
well. Some limited impacts are possible as far east as the
immediate I-35/35W corridor, but at this time, it`s likely to be
confined to mainly elevated surfaces such as bridges and
overpasses. Elsewhere, a cold rain is anticipated with little to
no travel impacts.
It will turn unseasonably colder across much of North and Central
Texas in the wake of the arctic cold front. Minimum wind chill
values or "feel-like temperatures" will fall into the teens for
much of the area on Wednesday and Thursday.
...Forecast Uncertainty...
Very cold air exists to the north of the Lone Star State and will
plunge southward over the next 12 to 24 hours. This airmass
SHOULD modify and warm as it heads southward and it`s unlikely
that the teens across southern Nebraska/northwest Kansas will make
it this far south. To what degree this airmass warms is a large
unknown and thus makes timing the onset of freezing rain
difficult. While possible, if the colder air is halted to the
north, little to no freezing rain may occur. This potential seems
to be withering, however, with successive model runs. There may be
a period in which some winter precip briefly changes back over to
a cold rain for parts of the area Thursday afternoon. There is a
chance for a second round of winter precipitation late Thursday
and into early Friday, mainly north of the HWY 380 corridor. Some
travel impacts due to snow/sleet accumulation MAY be possible
across these areas, depending on the amount of lift and available
moisture.
What IS known is that a cold rain should fall initially with an
eventual transition over to freezing rain. Freezing rain will
likely be the most dominant winter precipitation type late Tuesday
into Wednesday given the very shallow nature of the arctic
airmass. What is also known is that it WILL be cold across much
of North and Central Texas on Wednesday as most locales will
struggle to get out of the 30s for high temperatures.
...Meteorological Reasoning...
Late morning and afternoon surface analysis indicate that a very
shallow and cold airmass continues to spill southward over the
High and Central Plains. Temperatures, even just after the midday
hour were still in the teens! Pretty cold for this southerner!
Historically, model guidance has struggled with the speed of these
airmasses and so I`ve sided with the faster NAM/Canadian
solutions for temperatures on Wednesday. Strong and very deep
isentropic ascent is forecast to overspread much of the area late
Tuesday into Wednesday ahead of a stout PV anomaly analyzed across
the Pacific Southwest. The best ascent will actually be across
the Brazos Valley and East Texas and so PoPs are highest here.
Through Wednesday the better rain chances should encompass more of
the area and most locations will at least have a 60-80% chance for
precipitation. Ascent will be occurring over a very shallow and
cold airmass as the arctic front plows southward. Some locations
may see surface temperatures in the upper 20s with temperatures
around 4,000-5,000 feet AGL being in the upper 40s. This should
virtually ensure that no ice will be present below this level with
supercooled drops becoming probable closer to the surface. While
the latent heat release associated with freezing and the potential
heat exchange from falling water drops may encourage warming at
the surface, north winds of 10 to 12 knots may be sufficient to
keep a decent feed of colder air at the surface to promote waves
of freezing rain. The coldest air will likely be confined to
locations near/west of HWY 281.
The most likely area to see any sort of winter precipitation will
be for areas near and west of a Lampasas to D/FW to Gainesville
line. Most areas east of this line will simply see a cold rain
late Tuesday into Thursday.
Preliminary ice accumulations of one tenth to one quarter of an
inch appear reasonable and after discussion with WFO SJT, felt it
was prudent to highlight this threat with a Winter Storm Watch
that will go into effect around 0900 UTC Wednesday and run through
0000 UTC Thursday for parts of the area as noted above, impacts
to travel and infrastructure will be possible due to these
amounts. We advise individuals to NOT get locked into these
forecast accumulations (and focus on impacts as noted above) as
there is still the potential for some variability in time and
space.
As the PV anomaly swings eastward, this should induce strong low
level warm air advection which may result in a transition from
freezing rain back to a cold rain for parts of the area (likely
along the eastern fringes of the rain-freezing rain
line...probably near the I-35 corridor). Once this feature---the
PV anomaly---progresses northeastward, there will be the potential
for some banded precipitation. All indications at this time are
that the most probable locations will be near and north of the Red
River. However, the latest progs do show a band of 700-850mb FGEN
skirting the Red River. These types of banded precip can produce
brief bouts of heavy precipitation. By this time, the tropospheric
column should have cooled sufficiently to allow for a transition
to perhaps more of a snow/sleet mix. I`ve advertised this in the
worded forecast, mainly near and north of the HWY 380 corridor.
Some snow accumulation appears possible, but at this time, would
like to get a better look at the higher resolution model guidance.
In addition, a dry slot should be wrapping around the maturing
mid-latitude cyclone and depending on how quickly this feature
noses eastward will dictate any additional winter precipitation.
The good news for those that are not a big fan of winter weather
is that it should be short-lived as west winds overspread the area
on Friday and into this weekend. High temperatures should become
more seasonal with even above seasonal values possible on Saturday
and Sunday. The next chance for rain won`t arrive until late
Sunday and into early mid-week. There are questions revolving
around the moisture quantity, so for now, I`ve just broad-brushed
low PoPs.
Special thanks to surrounding offices (SJT, OUN and EWX) as well
as WPC for their coordination this afternoon.
Bain
&&
3 likes
The posts in this forum are NOT official forecast and should not be used as such. They are just the opinion of the poster and may or may not be backed by sound meteorological data. They are NOT endorsed by any professional institution or 
Yep, however it's been evident that the heat island is affecting some of the models, could make a big difference in a setup like this.
