Area Forecast Discussion
Issued by NWS Dallas/Fort Worth, TX

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FXUS64 KFWD 261245 CCA
AFDFWD

Area Forecast Discussion...CORRECTED
National Weather Service Fort Worth TX
745 AM CDT Mon Oct 26 2020


.SHORT TERM...
Update: The primary forecast concern (potential for some freezing
rain across our western zones tonight) remains unchanged with
this morning`s forecast issuance. Guidance is still in fairly good
agreement on timing/placement of freezing temperatures and no
changes were made to the Winter Weather Advisory. The only change
to products was to upgrade the Freeze Watch to a Freeze Warning,
still in the identical configuration to the Winter Weather
Advisory. For more on the freezing rain potential out west, see
the previous discussion included below.

One addendum to today`s forecast that will need to be monitored
is the potential for some convective sleet showers to impinge on
North Texas later this morning as deeper convection develops
across the northwest. While thermal profiles are still quite warm
(above freezing through the lowest 3 or 4 km), some ice pellets
originating above this layer may be able to survive the journey
to the surface if a quick sleet burst from a thunderstorm occurs.
However, surface temperatures are still expected to be around 40
degrees or higher during the day, so the sleet itself would not
pose any issues other than being an interesting distraction from
the otherwise monotonous light rain and drizzle which will consume
the entire area. No formal sleet mention has been included in the
forecast since this would be an infrequent occurrence, but we
will continue to mention isolated thunderstorms in parts of North
Texas this morning and afternoon.

-Stalley

Previous Discussion:
/Through Tuesday/

The main focus through the short term is the low-end potential
for some freezing rain across our western zones Monday night into
Tuesday morning. No changes were made to the existing Winter
Weather Advisory with this forecast issuance. However, a Freeze
Watch has been issued for the identical time and space as the
Winter Weather Advisory to advertise the potential for
temperatures to drop to or below freezing for the first time this
year. This forecast issuance has made heavy use of NAM/MET-based
guidance, with support from SREF/HREF data as well.

An arctic airmass continues to slowly seep southward into North
Texas, with temperatures now in the 40s across our northwestern
zones. Temperatures are generally expected to continue falling
over the next 24-36 hours as cold advection continues. With ample
moisture trapped beneath the frontal inversion, widespread
drizzle/mist and patchy fog will persist over most of the area
today and into tonight. Periods of enhanced isentropic ascent will
support elevated showers and even some isolated embedded
thunderstorms through the day as mid-level lapse rates steepen
with the approach of the deep upper low to the west. The most
widespread precipitation is expected to be across the northwestern
quadrant of the forecast area this morning and into the early
afternoon, before coverage decreases some later in the day.

The main window of concern through the forecast is late tonight
into Tuesday morning when temperatures may finally fall to near or
just below 32 across our northwestern zones. This would be most
likely roughly along and west of a line from Eastland to
Jacksboro. Latest short-term ensemble guidance as well as the
notoriously cold NAM/MET data suggests lows will only fall to
right around 32 or perhaps 31 at the lowest within this targeted
area by Tuesday morning. So, there is at least a low potential
for some freezing rain, but we have several factors working
against any type of significant winter weather within our CWA.

Precipitation tonight/Tuesday morning across the area of concern
will likely be somewhat convective in nature, elevated above
850mb within a pocket of steeper lapse rates. This means
precipitation rates will be higher than if rain was more
stratiform, and the higher rainfall rates would limit ice
accretion with the majority of model QPF wasted as runoff instead
of adding to ice accumulation. Another factor to consider (that
isn`t handled very well by models) is latent heat release at the
surface from any rain that does manage to freeze. The freezing
process requires heat release for the phase change, which may act
to keep the near-surface layer very near 32 or perhaps even just
above 32 degrees if any freezing is able to commence in the first
place. Typically, this process would need to be offset by strong
cold advection which would usher in a renewed supply of colder
air to offset warming via latent heat release. However,
temperature advection is expected to be fairly neutral during this
time period, with upstream temperatures only a couple degrees
colder. Lastly, precipitation will be falling from a cloud layer
within a deep inversion that will be nearly 50 degrees F. The
energy expended to cool 50-degree liquid water to 32 and
subsequently freeze it may be insurmountable by meager surface
temperatures that are only borderline freezing to begin with.

So, what all of this means for the Winter Weather Advisory area is
that the predominant precipitation type tonight into Tuesday is
expected to be cold rain. However, as temperatures briefly touch
31-32 near the surface, a window of some light ice accumulation
could occur, particularly on bridges, trees, and other elevated
surfaces. Overall, it appears that impacts would be minimal, with
perhaps only a few bridges/overpasses being impacted by freezing
rain accumulation. We will advertise ice accumulation totals of
as high as 0.05" in the official forecast, but the plethora of
factors discussed above does not warrant any higher totals or
worse impacts to be forecast at the present time. Buyer beware any
computer model graphics that depict ice totals equivalent to
forecast QPF, as model precip type schemes are not sophisticated
enough to account for all of the factors discussed above. For the
remainder of the forecast area, only cold rain (and perhaps a
rumble of thunder) is expected with no noteworthy impacts.

By Tuesday afternoon, some slight warming should occur and all of
the forecast area is expected to be above freezing by Tuesday
evening as occasional lighter rainfall/drizzle persists.

-Stalley

&&

.LONG TERM... /Issued 252 AM CDT Mon Oct 26 2020/
/Tuesday night through next Sunday/

A few more rounds of scattered to widespread drizzle/rain are in the
forecast Tuesday night and Wednesday before the dry weather returns
later in the week. While temperatures will drop a few degrees
Tuesday night, the freezing temps will stay far to the
west/northwest of our area. The highest rain chances will occur late
Tuesday night into Wednesday mainly across North Texas as the closed
upper low gradually moves eastward. We can`t rule a few rumbles of
thunder along the stronger precip bands.

A warming trend is expected to begin on Wednesday with highs around
6-10 degrees warmer than Tuesday. Rain chances will quickly end
Wednesday night, but the clouds might stay with us through Thursday
with some wrapped around moisture along the departing closed-low.
Dry weather is expected to prevail the rest of the week and into the
upcoming weekend with high temps in the 60s and 70s and overnight
lows mainly in the 40s.

Sanchez

&&

.AVIATION... /NEW/
/12z TAFs/

IFR/LIFR conditions will prevail at all airports for the
foreseeable future with periods of light rain and drizzle. Both
cigs and vsbys will be impacted, and will be at their lowest while
precipitation is occurring with VLIFR possible at times. TAFs
have have structured to identify time windows when the least
favorable conditions are likely to occur, although there is much
uncertainty, and irregular oscillations from IFR to VLIFR should
be expected during the next 24-36 hours. In addition, isolated
thunderstorms cannot be completely ruled out across North Texas
today, although this potential is low enough such that no TS has
been advertised in the TAFs for now. Otherwise, a north wind will
prevail at 10-15 kts, with perhaps some higher gusts during the
day.

-Stalley

&&


.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    53  39  46  46  53 /  90  90  60  70  80
Waco                60  41  49  48  57 /  90  70  50  70  60
Paris               52  44  50  50  59 /  90  90  40  60  80
Denton              49  36  44  43  53 / 100  90  70  70  80
McKinney            52  40  46  46  55 /  90  90  60  70  80
Dallas              54  41  49  49  56 /  90  90  60  70  80
Terrell             55  44  52  50  58 /  90  90  50  60  80
Corsicana           59  45  54  53  59 /  80  70  50  60  70
Temple              64  41  49  47  57 /  80  60  40  70  50
Mineral Wells       47  34  41  40  51 /  90  90  70  80  70

&&

.FWD WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 7 PM this evening to 1 PM CDT
Tuesday for TXZ100-101-115-116-129.

Freeze Warning from 7 PM this evening to 1 PM CDT Tuesday for
TXZ100-101-115-116-129.

&&

$$



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