Area Forecast Discussion
Issued by NWS Oklahoma City, OK

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925
FXUS64 KOUN 201556 AAA
AFDOUN

Area Forecast Discussion...UPDATED
National Weather Service Norman OK
1056 AM CDT Thu Apr 20 2017

.UPDATE...
Light rain and maybe a few storms will continue to be possible
this afternoon as the cold front continues to slowly move south.
It still looks like the more numerous and stronger storms won`t
occur until later this evening into Friday. Made some adjustments
to the forecast to show current trends. All updates out soon.

&&

.PREV DISCUSSION... /issued 611 AM CDT Thu Apr 20 2017/

AVIATION...20/12Z TAF Issuance...
MVFR to IFR stratus will continue to build southward across
Oklahoma, towards the Red River by mid-morning, in response to a
warm front moving into the region. Ceilings will continue to be a
challenge through the morning and the afternoon. Scattered showers
across north central Oklahoma will move across KPNC early in the
period. However, did not introduce SHRA to KOKC and KOUN mid to
late morning, but given radar trends, these sites may need to be
amended. Overnight, through the second half of the TAF period,
will be a challenge, as thunderstorm chances increase across the
region. Tried to keep the TAFs simple, using prevailing for many
sites through the end of the period, given confidence in
widespread thunderstorm activity across the region.

Kurtz

PREV DISCUSSION... /issued 426 AM CDT Thu Apr 20 2017/

DISCUSSION...
Primary forecast focus is on the evolution of thunderstorm chances
this morning through Friday, excessive rainfall/flooding and severe
chances.

This morning, isolated showers with some thunder have gradually
declined this morning along the frontal boundary, which currently
extends from southeastern Kansas to around the northeast Texas
Panhandle, per 20/08z surface analysis. The latest HRRR/RAP remain a
touch more aggressive with developing additional precipitation along
the boundary, while the various flavors of the WRF remain less than
impressed. This is likely due to the RAP and HRRR being more bullish
with the low level jet. Overall, still anticipate some isolated to
scattered development through sunrise, especially as the front
advances into north central and northeastern Oklahoma, but many will
remain dry.

Through the day, much of central and western Oklahoma will remain
dry, with the isolated shower chances holding on across central into
northeastern Oklahoma, as the surface front moves southward toward
the Red River through the afternoon. The front will stall near the
Red River Valley by the evening hours, as the initial 500mb trough
continues to advance eastward across the Great Lakes and upper
Midwest.

This evening, overall guidance remains in generally good agreement,
both synoptically and with general surface features. A subtle
leading short wave will proceed the deeper 500mb trough, which will
be traversing the intermountain west by the evening. In response,
isentropic ascent will increase through the evening and overnight
across the region, with the southern plains well tapped into the
Gulf via steady southerly return flow. This solution is relatively
unchanged from previous forecasts. through the overnight hours, a
relatively unstable airmass, driven by steep mid-level lapse rates,
will develop above the moist warm sector, spreading across portions
of western and central Oklahoma into the Texas Panhandle. As the
leading wave approaches late, after 9 to midnight, possibly,
increasing ascent and supported by strong mid-level westerlies
increasing effective shear, a few stronger updrafts may result in
large, damaging, hail productions.

Additionally, overnight the broad, stout, ascent will result in
scattered to numerous showers and thunderstorms developing, added by
top 10 percent precipitable water values (=>1.20in), increasing the
risk for heavy to excessive rainfall across much of central into
eastern Oklahoma, continuing into the early morning hours Friday.
The swath of heaviest rainfall will depend upon how far north the
frontal boundary lifts northward, as a warm front into the morning
Friday. Widespread amounts of 1 to 3 inches, with locally higher
amounts approaching 5, maybe 6 are possible. Did not make any
changes to current Flood Watch.

Into Friday, expect the cluster/MCS of ongoing convection across
portions of central and northern Oklahoma to be persistent. with a
continued risk for heavy rainfall, primarily long an axis from
central into northern and northeastern Oklahoma. Additionally, steep
midlevel lapse rates, decent elevated instability, and effective
shear, will continue to support a hail risk through the early
morning hours. General consensus from several CAMs, as eluded to in
the SPC Day 2, 3km NAM and the WRFs, would suggest a forward
propagation the MCS eastward, which would modulate the warm front
southward across southern Oklahoma. Obviously, this morning
convection will have a huge impact on what transpires in the
afternoon as the 500mb short wave trough finally arrives across the
southern plains by midday. However, given the gradual increase in
agreement, this southern displacement is plausible.

Into the afternoon, continued strong warm air advection across north
Texas into southern Oklahoma will result in a destabilized warm
sector developing, south of the warm front. A buffet of Bufr
profiles across southern Oklahoma into north Texas suggest modest
instability, 1000 to 2000 J/Kg, with deep layer shear supportive of
supercellular structure. However, given the current guidance, the
development of the MCS and it`s force of the warm front southward,
the opportunity, or window, for discrete convection appears to be
narrow and highly conditional. Given some consistency over the last
two days, initial storms may merge, becoming linear, building quickly
east and southeast across southern Oklahoma towards the Red River
and into north Texas by the evening and overnight as the cold front
surges eastward. This would suggest a potential wind and hail hazard.

As thunderstorms exit east and southeast early Saturday, from
southern Oklahoma, some lingering showers and convection may develop
on the backside of the exiting short wave in north central Oklahoma.
Overall, the weekend will be much cooler, with northerly flow
arriving. Highs on Saturday will be in the upper 50s for most, with
low to mid 60s across Texoma.

Kurtz

&&

.PRELIMINARY POINT TEMPS/POPS...
Oklahoma City OK  69  55  68  46 /  80  80  80  40
Hobart OK         68  55  73  45 /  20  60  50  20
Wichita Falls TX  80  61  80  48 /  20  40  30  40
Gage OK           70  52  58  41 /  10  70  70  20
Ponca City OK     69  53  58  45 /  60  70  80  60
Durant OK         80  65  79  51 /  20  20  30  70

&&

.OUN WATCHES/WARNINGS/ADVISORIES...
OK...Flood Watch from late tonight through Friday evening for OKZ007-
     008-012-013-018>020-024>031.

TX...None.
&&

$$

25/06



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