Area Forecast Discussion
Issued by NWS Oklahoma City, OK

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FXUS64 KOUN 300007

Area Forecast Discussion
National Weather Service Norman OK
707 PM CDT THU SEP 29 2016



VFR conditions will continue. Light northeast wind will become
light southeast Friday.


.PREV DISCUSSION... /issued 323 PM CDT THU SEP 29 2016/

Upper analysis shows a closed low to our east drifting slowly
south into Kentucky. We remain in a post frontal cool air mass
with temperatures below late September normals. The upper ridge
to our west will slowly deamplify through the weekend. A weak
shortwave trough on the periphery of the ridge will move into the
area by Saturday night bringing increased mid and upper level
clouds. Forecast soundings show dry low levels and weak
instability at best so the probability of measurable precipitation
is quite low. The main impact will probably be increase in mid
level clouds late Saturday through Saturday Night. Otherwise, no
significant impacts are expected through the weekend.

Medium range deterministic guidance has come into better agreement
with timing and track of the mid-upper low early next week,
however there is still considerable spread in the ensembles and
a lack of run to run consistency. Confidence in the forecast
early next week will increase if another run or two shows a
similar track and once the GFS ensemble spread decreases. ECMWF
has been the most consistent with the track and its ensembles
seem to be spread less given the more amplified and narrower
representation of the trough in its ensemble mean output compared
to yesterday. We reluctantly adjusted the end time of
precipitation upward with the growing consensus of a more
progressive system, but given the tendency for medium range
guidance to move these closed system systems along too fast, there
is concern that the precipitation event may linger across a
greater portion of our area into Wednesday.

Medium range guidance shows 12 hour 500 mb height falls ranging
from around 10-20 dm in the lee of the Rockies by late Tuesday and
this will result in a fairly significant surface response. A lee
low will rapidly form, probably somewhere over the eastern
Colorado Plains (still some uncertainty on position given
aforementioned model disagreements). We have adjusted winds up in
the blend closer to statistical guidance on Tuesday to account for
the expected rapid deepening of the surface low.

Short wavelength in upper flow pattern will continue into early
week. Due to this and fairly quick progression of the upper
trough, a long duration fetch of undisturbed moist gulf air mass
is not expected. The return of moisture may not be that robust in
this pattern but should be sufficient for a narrow corridor of at
least weak instability. As strong ascent overspreads the modest
instability axis Tuesday afternoon, thunderstorms should form.
More than sufficient deep layer shear will be present for storm
organization/persistence. Severe thunderstorms are possible,
although resolution of mesoscale detail will be needed for
confidence in the severe weather threat to increase. It is too
early to be specific about potential convective modes and hazards,
but given amplified nature of the trough and strong meridional
flow present as intense mid-level speed max rounds the base of
the trough which becomes increasingly parallel to the Pacific
front, a trend toward linear convection may occur late in the day
or during the evening. We will wait for medium range model run to
run consistency to stabilize and ensemble spread to lessen before
addressing convective mode, hazard types, initiation area, and
timing Tuesday afternoon.

Concerns that the medium range models may be too progressive
continue through mid week into late next week. For now, a
consensus was accepted, and the forecast was kept dry with
temperatures trending downward to near or just below early
October normals.



Oklahoma City OK  51  74  53  80 /   0   0   0   0
Hobart OK         50  76  53  81 /   0   0   0   0
Wichita Falls TX  51  77  54  83 /   0   0   0   0
Gage OK           46  78  52  81 /   0   0   0  10
Ponca City OK     47  74  50  78 /   0   0   0   0
Durant OK         52  78  54  83 /   0   0   0   0


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