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Area forecast discussion 
National Weather Service Fort Worth Texas
1121 PM CST Thursday Feb 26 2015

Aviation...
only changes to this taf set are to bring visby down to 3sm in
the snow Friday...and add IFR ceilings/visby early morning
Saturday in the -fzdz. Evening guidance has quantitative precipitation forecast and snow
accumulations pretty similar to previous runs and we still expect
1" to 2" in the metroplex. 9 am start time for snow is still
indicated.

I have less confidence on timing of changeover to -fzdz but timed
it based on forecast soundings that showed the dendritic growth
region drying out by middle evening. My guess is that as long as
there are dendrites growing...they are very good at accreting out
any low level supercooled water (br/dz drops) and the fzdz should
not occur until the dendrites stop forming. There may be some
sleet or grainy snow mixed in during the changeover as well.
Latest guidance still shows surface temperatures climbing past freezing
by noon Saturday...when changeover to -dz/-ra will occur. 84

&&

Update...
with a very complex forecast in place regarding timing and exact
location where snow bands set up Friday morning...have decided that
confidence with current forecast is good and no changes were made.
Cannot rule snow possibly entering far western counties just before
daybreak...but there is plenty of dry air below 800mb on tonight/S
forward sounding to overcome and decided it will take time for top-down
saturation. Latest high-res models of the hrrr, WRF, and NAM models
do show about a 1-2 hour sooner arrival of the snow into the I-35
corridor from the dfw metropolitan and points north...as the band sets up
in an east-west orientation.

At this time...the morning rush should be 'ok'...but snow will
likely arrive by 9 am...if not between 8-9 am in the western metropolitan
and points north and west. This system does show some modest
elevated instability. So any convective snow bands or slight shift
north or south could have quite an effect on location...amounts
and impacts along and north of I-20 on Friday.

00z models are just now flowing in...so will allow the overnight
forecasters to make any adjustments in location...amounts and
impacts with the Winter Weather Advisory.

05/

&&

Previous discussion... /issued 401 PM CST Thursday Feb 26 2015/
..Winter Weather Advisory issued for the northwest half of the
County Warning Area including the dfw metroplex...

Snow band forced via frontogenesis finally waned early this
afternoon with highest snow totals of 1-2 inches from Young County
into Palo Pinto County. While temperatures are near or below freezing in
this area...sheriff offices report that snow has been steadily
clearing off of the Road...likely subliming as dry north winds
bring dewpoints down into the teens over the region. The cold and
dry advection will continue into the overnight hours and will help
temperatures fall into the 20s areawide despite extensive cloud cover.

Texas remains at the base of a broad longwave trough over the
country and as a result Arctic air will continue to pour southward
through the plains with our thermal profile becoming unusually
cold aloft. Meanwhile a fast moving shortwave that is currently
located near The Four Corners region will track through the region
tomorrow. For an open shortwave trough...it has an impressive
appearance on todays water vapor loop and it certainly raises
confidence that strong lift will overspread North Texas by
daybreak. Thermal profiles will be very cold and there is no
question that precipitation will be in the form of snow. In fact
the only concern or mitigating factor with snow is going to be
very dry air in the lower levels. Climatology of snow patterns
says these systems often never produce more than 4 inches of snow
because they are unable to grab any quality low level moisture
from the Gulf since the 850mb winds never get an opportunity to
turn to the south. Thus the snow that is produced aloft will have
to work through and saturate the dry layer before it can reach the
ground. How much snow this dry layer will eat up is a essentially
an unanswerable question...but it is a big limiting factor.

Despite the moisture concern...I dont want to imply that I am not
impressed at the setup at hand. This system will have very cold
air in place with saturation from -10c to -20c which is the
favorable zone for snow dendrites...and will help make snow
flakes grow fast and large. In addition...steep lapse rates of 6
to 7 degree/km exist from 400-600mb which adds an element of
convective instability and extra vertical depth for snow to grow.
Given the cold temperatures in place and favorable snow making
microphysics environment...an unusually high snowfall to liquid
water ratio is expected. This snow will not be heavy and wet like
yesterday...but more dry and fluffy.

Since the snow is driven by upward dynamic forcing...the track of
the shortwave will determine where the snow falls. Because this
system pivots through the northwest half of the region...this is
the area that accumulating snow is most likely. An advisory was
issued along and northwest of a line from Comanche to Dallas to
Cooper. For the northwestern part of the advisory...northwest of a
Eastland to Gainesville line...have gone 2 to 4 inches of snow
which is on the higher side of the guidance envelope. As you head
farther southeast across the County Warning Area...snow amounts will taper off to
just a dusting. For the metroplex the current forecast is for 0.5
to 1.5 inches with the lower amounts being in southeast Dallas
County. I must stress that just an error of 0.1 inches of liquid
quantitative precipitation forecast may balloon out to an extra 2 inches of snowfall or result in
way less than forecast too.

High temperatures tomorrow will be below freezing for the entire
area...but in areas where snow falls...temperatures will likely drop
into the 20s. This means we dont expect a lot of melting when it
hits the ground or streets. It is really hard to know if travel
impacts will begin at a half inch or 1 inch or 2 inches. There is
the potential of travel impacts on bridges and overpasses in the
metroplex at even a half inch...which is why we felt it was best
to include them in the advisory for a seemingly low amount of
snow...that plus the inherent uncertainties in forecasting snow
amounts in the first place! If the band of snow does track too
far northwest to affect the metroplex...the winter event
continues into Friday night and Saturday so it would just serve
as extra lead time for that.

After the shortwave passes Friday afternoon...winds just above the
surface become more southeasterly in response to a digging trough
on the West Coast. These winds will start a strong warm/moist
advection pattern with isentropic lift organizing over the entire
region. This lift will be sustained from Friday night through
Sunday...with drizzle and light showers likely across North Texas.
MOS probability of precipitation look much too low given the strength and persistence of
this signal. The issue is that temperatures will still be below
freezing at the surface into Saturday. And while the entire column
is below freezing Friday night...the precipitation type could
switch from light snow grains to freezing drizzle. Frozen
precipitation requires an ice nuclei because water does not know
how to form into the Crystal structure without help. Pure Water
does not freeze until -40c...but in real world clouds...pollen and
dirt particles will /teach/ water how to freeze at temperatures of -8c.
So once temperatures in the cloud rise above -8c but stay below
freezing...there will no longer be ice in the cloud...and
precipitation will be super cooled raindrops which freeze on
contact with the surface. This type of freezing rain/drizzle is of
the highest impact because it is so efficient at going to ice
when it hits the surface. As a result we are concerned that
bridges and overpasses will receive a glaze of ice Friday night
into Saturday. Until temperatures rise above freezing Saturday
afternoon...this will be a big concern for travel. We expect most
of the County Warning Area...with perhaps the exception of the southeast zones to
contend with this freezing precipitation. The Winter Weather Advisory
will likely be expanded to include additional counties during the
next 24 hours.

Once temperatures rise above freezing Saturday night with the approach
and passage of a warm front...just a chance of rain showers are
expected Sunday. The front will slosh back to the south on Monday
with cooler temperatures again...but they will remain above freezing. The
chance of showers near and behind the front will persist. On
Tuesday a strong shortwave trough will move through the plains and
bring much warmer Gulf air into the region with highs in the 70s.
This will result in some surface instability...and perhaps a
severe weather threat when the next front moves through Tuesday.
The SW flow behind the front may mean it is not until Wednesday
that the rain and clouds can be cleared from the forecast...but
eventually drier weather will prevail by middle-late week.

Tr.92




&&



Preliminary point temps/pops...
Dallas-ft. Worth, Texas 26 32 28 38 37 / 5 60 60 60 40
Waco, Texas 28 35 30 40 39 / 5 20 40 60 40
Paris, Texas 23 30 26 38 37 / 5 50 60 60 40
Denton, Texas 24 30 26 36 35 / 5 70 60 60 40
McKinney, Texas 25 31 27 38 37 / 5 60 60 60 40
Dallas, Texas 26 32 29 39 38 / 5 60 50 60 40
Terrell, Texas 27 33 29 40 39 / 5 40 40 60 40
Corsicana, Texas 28 35 30 42 41 / 5 20 40 60 40
Temple, Texas 28 36 32 41 40 / 5 20 50 60 40
Mineral Wells, Texas 25 29 26 35 34 / 10 70 60 60 40

&&

Forward watches/warnings/advisories...
Winter Weather Advisory from 6 am Friday to 6 PM CST Saturday
for txz091>095-100>106-115>120-129>133-141.



&&

$$

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