Area Forecast Discussion
Issued by NWS Oklahoma City, OK

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000
FXUS64 KOUN 181525 AAA
AFDOUN

Area Forecast Discussion...UPDATED
National Weather Service Norman OK
1025 AM CDT Thu May 18 2017

.UPDATE...
Here is a mesoscale update.

&&

.DISCUSSION...
Determining exactly when and where storms occur today remains a
challenge. Latest guidance overall has been suggesting an earlier
start for storm initiation with first storms forming across
southwestern Oklahoma and western north Texas around noon.

Overall, based on latest model guidance (particularly the
HRRR/RAP) and incoming data from a special 15 UTC KOUN sounding,
confidence is growing that storms will form earlier and somewhere
over southwestern Oklahoma or western north Texas around noon
today. The 15 UTC KOUN sounding depicted that the base of the low
level inversion has lifted about 40 mb (from around 900 mb to 860
mb) and low level moisture deepening below the inversion. Based on
latest HRRR/RAP guidance, this lifting is expected to continue in
the next few hours, which would allow for storms to form earlier
than previously expected once surface temperatures reach the
lower to mid 80s near or just east of the dryline across
southwestern Oklahoma and western north Texas. A weak mid level
disturbance may be causing the lift, though the exact cause for
the lift remains uncertain.

If these storms form around noon in southwestern Oklahoma and
western north Texas, they may at first be slighlty elevated with
giant hail as the main hazard. However, these storms would likely
become surface-based due to daytime heating no later than 3 pm as
they move northeast. This could increase the tornado potential
across western north Texas, southwestern and central Oklahoma this
afternoon and evening while slightly decrease the tornado
potential in northwestern Oklahoma. Again, the exact details
remain uncertain today.

The bottom line is that the combination of shear, instability,
and moisture support significant severe storms with giant hail
and tornadoes this afternoon and evening somewhere over the
western two thirds of Oklahoma and western north Texas.

MBS

&&

.PREV DISCUSSION... /issued 639 AM CDT Thu May 18 2017/

DISCUSSION...
The 12Z aviation discussion follows....

AVIATION...
A large, powerful storm system will move through the southern
Plains today and tomorrow. A jagged deck of stratus will cover
much of central and eastern Oklahoma (also the Wichita Falls area
in Texas) this morning, then break up toward 18Z. Intense/severe
thunderstorms are forecast to develop quickly during the early
afternoon, probably between 19Z and 20Z, across western Oklahoma
and adjacent parts of Texas. The initial storms will mainly affect
KLAW, KCSM, KHBR, and KSPS, then spread/move east/northeast with
time. Then, a cold front will enter northern Oklahoma around
sunrise on Friday, bringing a minor wind shift and possibly some
very low stratus to far northern Oklahoma. At the same time, a
renewed deck of stratus will be advancing north across the Red
River.

CmS

PREV DISCUSSION... /issued 342 AM CDT Thu May 18 2017/

DISCUSSION...
The second shortwave trough in this week`s series is rounding the
base of the longwave trough, and is now centered near Salt Lake
City, Utah. At the same time, a surge in low-level moisture is
advancing quickly north across Oklahoma. High dewpoint
temperatures have extended to Ponca City, Oklahoma City, and
Lawton, and this humidification will extend across most of the
remainder of Oklahoma (and western north Texas) by this
afternoon.

The HRRRX model has been fairly consistent in breaking out the
initial convection early this afternoon over Texas, in the general
vicinity of Abilene, then spreading/expanding this area rapidly
north through the afternoon. In a general sense, this is probably
what will happen. Supercell thunderstorms are likely within the
expected environment, and the SPC has issued a "High Risk" of
severe storms for today for much of the northwest quadrant of
Oklahoma, with lesser risk areas covering the remainder of our
forecast area.

A cold front will move into northern Oklahoma on Friday, and by
evening, it should be near I-44. This will limit the risk of
severe storms to the north, but along and south of the front,
there will again be a risk of severe storms. There is more
question about what will happen tomorrow, since lingering early-
morning convection will alter the environment in unpredictable
ways.

Rainfall tomorrow may be quite heavy near the front. There is
still some uncertainty where the heaviest rain may fall, and
whether it will be enough to cause more than localized flooding,
so there is not yet justification for a flood watch. Nevertheless,
some urban and small stream flooding seems likely, if the expected
scenario is correct.

The front will continue its advance on Saturday and Sunday, as
rain/storm chances slowly diminish from the northwest. This will
bring a relatively quiet period for most of the weekend and into
early next week. Another wave sweeping south across the Plains
will, however, bring another chance of showers and thunderstorms
(with a lower risk of severe storms) for Tuesday and parts of
surrounding days.

&&

.PRELIMINARY POINT TEMPS/POPS...
Oklahoma City OK  85  66  78  58 /  60  60  80  90
Hobart OK         85  65  82  55 /  60  50  70  70
Wichita Falls TX  89  69  84  61 /  60  40  70  80
Gage OK           83  57  79  48 /  60  60  40  30
Ponca City OK     84  65  78  56 /  30  70  70  80
Durant OK         86  69  83  66 /  30  40  40  70

&&

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

$$

17/06



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