Area Forecast Discussion
Issued by NWS Oklahoma City, OK

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FXUS64 KOUN 222052

Area Forecast Discussion
National Weather Service Norman OK
352 PM CDT MON AUG 22 2016

Primary challenge for this forecast continues to be spatial
and temporal details of precipitation chances through the week.

Water vapor loop and RAP analysis shows a closed low located over
Alberta moving slowly southeast. Broad weak troughing is present
across the western CONUS with weak ridging in the east. This places
the southern plains in weak mean southwesterly flow. Embedded within
this larger/broad trough are a series of shortwave troughs that are
identifiable in water vapor impinging on the central High Plains.

In response to this first shortwave, a deepening lee surface trough
and responding strengthening south/southeasterly surface winds have
been seen in the observations in the High Plains. Low level response
across our area so far has been modest. Some strengthening of
surface winds this morning brought moisture northward into
lingering anomalously cool/dry post frontal environment this
morning resulted in areas of advection fog across the southeast
portion of the area. Dew points in the low to mid 70s have already
moved back into south- central Oklahoma.

This northward moisture surge/advancing warm front will result in
deeper moisture and increasing instability later tonight. Forecast
sounding profiles show modest instability developing and as low
level jet strengthens, some convection should develop overnight.
Details on how this evolves is not particularly clear since there is
not a well-defined area of moisture convergence or relatively
concentrated ascent on isentropic surfaces in short term model
guidance. Therefore, coverage may be limited and the potential may
exist over a fairly broad area over primarily the south or southwest
half of the forecast area.

Fog seems less likely tonight unless the warm advection process is a
little slower and northwestern sections cool more than expected. A
brief period of reduced visibilities may occur if timing is optimal
on the leading edge of the more substantial moisture late tonight as
it continues moving northward. Low stratus is more likely than fog
in this scenario.

We will need to make adjustments based on trends but it appears
there may be a period of weak ridging and subsidence in the wake of
the lead shortwave to most of the convection late tomorrow and
tomorrow evening. We have added a little more detail to try and
account for this with a period of lower probabilities before the
next wave arrives late in the night or early Wednesday necessitating
a slight increase in probabilities across the west. The best chance
should come late Wednesday or Wednesday night as a more substantial
shortwave trough moves into the area and forces a cold front into
northwest Oklahoma. Convergence along this front in a moist and
fairly unstable environment should support decent coverage of
storms, some of which could be fairly robust given increase in 0-6km
bulk shear to around 30 knots and possibly higher later in the
evening. Forecast soundings suggest slightly cooler mid-levels and
MLCAPE around 1,500 J/KG. In addition to what looks like a minor
severe weather threat with marginally severe hail and wind, locally
heavy rainfall amounts and possibly localized flooding is another
concern given PWAT values of at least 1.5 inches and relatively slow
storm motions.

Precipitation chances continue late in the week, possibly spreading
a little further south and east as the deamplifying shortwave lifts
northeast and southeast upper ridge starts moving east. Confidence
is low for the weekend given the spread of ensemble medium range
guidance and differences in deterministic guidance. ECMWF is more
amplified and stronger with a trough that would impact the area GFS
is more uniform with southwesterly mean flow with neutral to
slightly increasing mid-level heights during that period. We
retained low probabilities and will refine later once guidance comes
more in agreement.



Oklahoma City OK  71  88  74  93 /  40  50  20  10
Hobart OK         72  90  73  93 /  40  40  30  20
Wichita Falls TX  72  91  73  95 /  40  30  20  10
Gage OK           69  92  72  92 /  20  20  30  40
Ponca City OK     73  89  75  93 /  20  50  20  20
Durant OK         72  92  73  95 /  30  20  10  10


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