Area Forecast Discussion
Issued by NWS Tulsa, OK

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FXUS64 KTSA 201144

Area Forecast Discussion
National Weather Service Tulsa OK
644 AM CDT Fri Oct 20 2017

Scattered to broken mid and high level cloud decks are expected
across Eastern Oklahoma and Northwest Arkansas today with
southerly winds becoming breezy/gusty this afternoon. Gusts in the
20-25+kt range will be possible over Northeast Oklahoma. This
evening and overnight...mid clouds are forecast to become
broken/overcast from west to east with the potential of MVFR
ceilings developing late tonight over Eastern Oklahoma. For now
will introduce tempo groups for BVO/TUL/RVS/MLC and keep ceilings
just within the VFR category for Northwest Arkansas sites. Also
overnight...southerly winds look to remain breezy with the
potential of LLWS across the CWA.


.PREV DISCUSSION... /Issued 326 AM CDT Fri Oct 20 2017/


The main points of emphasis in this morning`s forecast will be: 1)
Shower/storm chances today and tonight with a lead shortwave
trough, 2) Storm severity and heavy rainfall potential with the
second and stronger trailing shortwave trough and associated cold
front late Saturday and Saturday night, and finally 3) Precip
chances with the front late next week.

A southerly flow prevails aloft over OK/AR this morning,
downstream from a shortwave trough and PV max near El Paso. Ahead
of the shortwave, a tongue of higher PWAT air is nosing up into
southern TX. This moisture should continue to stream farther north
today, and interact with forcing for ascent from the shortwave to
produce storms over eastern TX later today. Some of this activity
may lift up into SE OK before dissipation this evening. Another
challenge is precip chances tonight. Forcing from the wave should
be lifting north and east of our area tonight. Mid level
isentropic analysis suggests some lift across my northeast areas.
The short term hi-res models are not excited at all, the ECMWF
shows only patches of very light qpf. The GFS shows more QPF in
the northeast areas possibly due to the reason mentioned earlier.
All that said, will leave low PoPs in the east/northeast for
tonight given low confidence of occurrence.

Attention now turns to the potent and progressive shortwave trough
now moving onshore from the Pacific. This system is located near
the nose of a powerful cross-Pacific upper jet. Models suggest
that the shortwave energy will split in two as it moves toward the
Plains, partly due to buckling of the jet over the Pacific and
subsequent development of a West Coast ridge axis. The southern
piece of energy, at the basal portion of the parent trough, will
slide across the central/southern Plains this weekend. An
associated Pacific cold front will interact with a moistening and
destabilizing warm sector Saturday and Saturday night. Storms will
likely fire first over western/northern OK during the afternoon and
then quickly grow upscale into a squall line and march east across
our area Saturday night. Damaging winds will be the primary threat
in our area, and probably thru around 06Z before activity begins
to wane as the orientation of the upper flow becomes more parallel
or even anafrontal in relation to the surging front. If we`re
going to see any QLCS spin-ups it would have to be early on as the
activity is entering eastern OK when the shear vector orientation
is more favorable. Though the ECMWF trended down slightly with
the 00z run, models still show decent QPF with the frontal
convection, between 1 and 2 inches over much of the area. The
progressive nature of the system suggests that the flash flood
threat will be localized at best. However, the widespread nature
of the rains could cause some rivers to flood.

The buckling of the cross-Pacific jet will result in a highly
amplified upper flow pattern over the CONUS next week. A polar
cold front will push thru late Monday and Monday night, but will
not have moisture to work with. After a noticeably cooler day
Tuesday, temps will rebound quickly as downslope flow prevails in
between shortwave troughs. The next upper system will dive
southeast over the Plains late next week and will force another
cold front thru the area. Moisture availability is in question at
this point, and have thus elected to maintain a dry forecast for



TUL   78  66  78  53 /  10  10  30 100
FSM   79  63  80  60 /  10  20  20 100
MLC   79  66  78  57 /  10  10  30 100
BVO   78  65  77  51 /  10  10  40 100
FYV   75  61  77  56 /  10  20  20 100
BYV   77  58  77  56 /  10  20  10 100
MKO   79  63  78  54 /  10  10  30 100
MIO   78  62  77  51 /  10  20  20 100
F10   78  65  77  54 /  10  10  30 100
HHW   78  64  81  60 /  30  10  30 100




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