Area Forecast Discussion
Issued by NWS Oklahoma City, OK

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FXUS64 KOUN 191518
AFDOUN

Area Forecast Discussion
National Weather Service Norman OK
918 AM CST Mon Feb 19 2018

.UPDATE...
Significant changes were made to the forecast today through
tomorrow morning for the northern half of Oklahoma based on
current front movement. 12Z nam/wrf have caught on to the fact
and were followed closely with the RAP. This raises uncertainty
to POP for next 12 hours but main changes where to winds,
temperatures and dewpoints. The ongoing precipitation this morning
may gradually end or move northeast with better chances again
tonight into Tuesday.


&&

.PREV DISCUSSION... /issued 544 AM CST Mon Feb 19 2018/

..Updated Aviation Discussion...

AVIATION...
For the 12z TAFs:

MVFR ceilings at airports across central and southern Oklahoma and
north Texas may become VFR this afternoon before chances of MVFR
or lower return this evening. Gusty south winds will continue.
Winds are expected to shift to northerly at airports across
northern Oklahoma this morning as a front moves through and stalls
to the south of GAG/WWR/PNC. Winds will eventually become easterly
and then southeasterly tonight in response to a strengthening low
in Colorado. There`s not a particular focus for showers and
thunderstorms so coverage and timing is difficult for this set of
TAFs. Southern and eastern TAF sites could be impacted by a shower
or thunderstorm at any time but the probability is fairly low
until later tonight when an uptick in activity is expected.

PREV DISCUSSION... /issued 415 AM CST Mon Feb 19 2018/

DISCUSSION...
Short term forecast challenges mainly center around boundary layer
moisture characteristics, dryline position, and
coverage/timing/severity of convection today. Other forecast
concerns include elevated to near critical fire weather across
western Oklahoma and western north Texas this afternoon, heavy
rainfall amounts across the southeast half of the forecast area
through Tuesday, and potential for a minor winter weather Wednesday
into Wednesday night. See details below.

On the synoptic scale, the flow pattern is quite amplified with
impressive mid-level height anomalies along the east coast and low
height anomalies centered over the Great Basin region. The eastern
ridge and mean western troughing pattern will continue for the next
several days. Periodic shortwave troughs will move through broad
southwesterly flow over the central part of the country providing
enhanced periods of ascent across our area. Timing these out to
assign highest precipitation probabilities is challenging.

Elevated convection should increase through the early morning within
warm conveyor/strong low-level jet in response to mid-level
moistening and weak elevated instability. Confidence is high enough
to support rather high probabilities this morning. Last evening`s
sounding here in Norman showed a saturated boundary layer sufficient
for drizzle, trapped beneath capping warm/dry layer, and trends
since then have been for drying of the boundary layer and
diminishing drizzle as the warm front has shifted north. Models are
not consistent with how the structure of remnant EML/capping
inversion in warm sector (~850 hPa) and boundary layer moisture
below that will evolve. Consequently there is a fairly large spread
with convective coverage and timing among a variety of CAMs.

Current thinking is that dryline will mix eastward across southwest
Oklahoma, possibly as far east as El Reno to Lawton by peak mixing
in late afternoon, but likely a little west of there (short term
model guidance varies considerably on dryline position). To account
for the uncertainty we have highest chances closer to I-35 corridor
and relatively low chances across southwest Oklahoma. This won`t be
a particularly sharp/strong dryline and convergence along it is
weak. Nevertheless, is should be the western terminus of convective
potential during the afternoon. To the east of the dryline, and
south of a cold front which will be oriented west to east across
northern Oklahoma, some diabatic surface heating may occur and at
least modest surface based instability is expected to develop.
Stronger instability would depend on how much stratus clears, but
the general pattern (now deeply in the warm sector) should support
less extensive stratus than yesterday. Latest HRRR and RAP forecast
soundings support this and consequently both models have trended
higher with SBCAPE. It seems increasingly likely that around 1,000-
1,500 J/kg SBCAPE will materialize by late afternoon. This in
combination with ~45 knots of effective shear will support organized
fairly robust updrafts and the potential for some severe
thunderstorms with large hail and damaging winds the main threat.
The tornado threat should be limited by unidirectional shear
profiles. If convection does initiate and can sustain intensity and
remain mostly surface based into the evening, backing low level flow
in response to deepening lee low in eastern Colorado could enhanced
low level shear across portions of central Oklahoma and we`ll need
to watch this closely.

Another area of interest will be just north of the aforementioned
cold front. Forecast soundings suggest elevated convection potential
with at least weak instability from elevated parcels at the top of
the frontal layer. Coverage could be more numerous than further
south due to ascent atop the cold post-frontal air mass to its
north.

One passing shortwave to the west tonight and enhancement of upper
jet entrance region to our north will help force a fairly
significant uptick in convective coverage within warm conveyor.
Instability should be weak given deep saturated profiles and weak
mid-level lapse rates, as well as stabilizing boundary layer. Any
severe threat should diminish by mid-late evening. Nevertheless,
weak instability in an unusually moist environment for mid
February (PWAT values ranging from 1.25" in central Oklahoma to
1.75" in southeast Oklahoma) will support locally heavy rainfall.
Heavy rainfall potential will continue across the southeast half
of the area Tuesday before broad trough opens/weakens and shifts
northeast Tuesday forcing another cold front through by evening.
Upstream air mass is quite cold and sub-freezing temperatures are
expected to move into most of the area. By then, deep moist axis
and precipitation will have shifted east.

By early Wednesday the next in a series of shortwave troughs will be
moving toward our area from New Mexico. In response, 850 hPa flow
will veer and pull moisture northward atop existing cold air mass.
Isentropic ascent scenario is likely to develop Wednesday with light
precipitation potential. We have kept precipitation probabilities
low Wednesday during the day since the low levels could be quite dry
and depth of saturation varies among model guidance. By Wednesday
evening and especially overnight, saturation should be sufficient
for precipitation across much of the area. Temperatures may still be
below freezing if the shallow cold air mass isn`t removed by the
warm advection process. There is a lot of uncertainty and we`ll need
to see first if the magnitude of cold air mass is as significant as
models currently suggest, and second if residual cold/sub-freezing
air can last as long as it takes for deep saturation and
precipitation generation. Current indications are that some freezing
rain will probably occur across portions of the area. Closer to the
Red River across southern Oklahoma, surface temperatures will
probably be warm enough for just a cold rain. We`ll watch trends as
this has some potential to be impactful across portions of central
and northern Oklahoma.

By Thursday, although some wintry precipitation could linger across
northern Oklahoma, warm advection should push temperatures upward
enough for rain across most of the area. Deepening broadening
persistent troughing to the west will be favorable for fairly high
rain chances through Saturday. A drier weather pattern looks to
develop by the coming weekend as troughs stay at higher latitude and
overall pattern is less amplified.

BRB

FIRE WEATHER...
Elevated fire weather conditions may develop this afternoon across
western portions of Oklahoma and western north Texas behind a
dryline. There is some uncertainty how far east the dryline will
advance, but at least a small section of sufficient RH/winds are
expected for elevated fire weather concerns. These conditions
should best align across areas that received decent rainfall
Friday night, and so fuels may not be quite as anomalously dry as
previous weeks. Latest estimates suggest ERC-G values are on the
high end of normal or around the 75th percentile. Given still
fairly dry and dormant fuels and the expected meteorological
conditions, we will issue a Fire Danger Statement to highlight the
threat.

A small area of elevated fire weather conditions may redevelop
tomorrow in western north Texas, particularly if winds are
stronger than currently expected.

A cold front will move through Tuesday afternoon and evening
bringing a shift in the wind to northerly and colder temperatures
that will limit the fire potential for at least a few days.

BRB

&&

.PRELIMINARY POINT TEMPS/POPS...
Oklahoma City OK  70  64  68  27 /  60  70  60  10
Hobart OK         77  61  66  26 /  20  30  20   0
Wichita Falls TX  75  66  72  33 /  30  60  50  20
Gage OK           59  39  45  20 /  10  10  10   0
Ponca City OK     66  39  50  24 /  70  60  40  10
Durant OK         74  65  66  39 /  60  80  90  70

&&

.OUN WATCHES/WARNINGS/ADVISORIES...
OK...None.
TX...None.
&&

$$



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