Area Forecast Discussion
Issued by NWS Oklahoma City, OK

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FXUS64 KOUN 200827
AFDOUN

Area Forecast Discussion
National Weather Service Norman OK
327 AM CDT Tue Jun 20 2017

.DISCUSSION...
Looping imagery from GOES-16 7.34/6.95 water vapor bands clearly
show dry/subsident post-frontal air mass across the southern
Plains. Latest regional surface observations show winds have
become easterly and are still light. T/Td depressions are quite
low across eastern and southern Oklahoma. A few observations from
the Ouachita Mountains westward along the Red River in southeast
Oklahoma have had visibility reductions. The only surface
observations in our area showing visibility reductions as of this
writing is Durant and Ardmore. Locations possibly as far west as
Wichita Falls and Lawton could see light fog early this morning,
but the most likely locations will be over south-central Oklahoma
where rainfall occurred last night.

Light generally non-impactful fog is the main near-term forecast
challenge, aside for perhaps some low stratus over our north
Texas counties and into southwest Oklahoma (see previous aviation
discussion).

A large upper high with anomalous 500 mb heights is expected to
build slightly eastward today and this will support warmer
temperatures than we saw yesterday. Low to mid 90s will be common
for highs across our area. Weak north-northwesterly flow aloft on
the northeast periphery of aforementioned high may steer some
convection into far northwest Oklahoma this evening. A well
mixed/hot boundary layer over the central High Plains should
promote sufficient instability for thunderstorms to develop by
afternoon. Most of the deep layer shear for storm organization is
directional, but 25-30 knots of mean mid-level flow should be
enough for storms to at least approach northwest Oklahoma before
diminishing late in the evening. Storm motions should be slow, but
this could be augmented by cold pool development if storms are
extensive enough. Instability should be fairly weak so severe
thunderstorms are unlikely. Especially since well-mixed boundary
layer should be stabilizing by the time the storms reach our area,
although thermodynamic profiles in forecast soundings support
strong wind gusts with any reasonably robust convection that can
make it into the area.

A similar scenario is expected Wednesday. Thunderstorms may impact
northwest Oklahoma during the evening, and perhaps Thursday
evening too. During this period the synoptic scale pattern will
change little. The exception is a shortwave trough traversing the
central Canadian provinces on Wednesday. The southern extent of
this trough over the central Rockies may tend to flattened the
Southwest mean ridge slightly and provide enough forcing for
ascent for perhaps greater convective coverage and maybe a better
chance of a sustained MCS that may move toward our area. We have
higher precipitation probabilities over northwest Oklahoma for
this reason.

There could be a low-end fire threat Wednesday afternoon (see Fire
Weather section below).

Meanwhile, potential tropical cyclone in the Gulf will be moving
north-northwestward generally paralleling the Sabine River Valley.
The exact track is uncertain but a more westward path could impact
our temperature forecast as clouds may be more extensive.
Subsidence on the outer periphery may limit convective potential
Thursday evening if the system is far enough west. We may need to
increase probabilities across the far southeast portion of the
area depending on trends.

On Friday, remnant tropical system should lift northeast. We`ll
still have northwesterly flow aloft so any small and difficult to
resolve perturbation in the flow may increase chances of
convection across the area, although background large scale mid-
level height rises should work against that. Not overly excited
about precipitation chances late Friday/Friday night, but for
consistency and collaboration sake we retained chance
probabilities.

Some fairly significant discrepancies in deterministic guidance
is seen Friday into Saturday. Compared to ECMWF, GFS is quicker
with a shortwave trough traversing the northern Plains and Midwest
which sends a cold front through the area. There is about 24
hours difference. For this reason, Saturday is the day with the
least certainty. We have weighted closer to the ECMWF which has
support from a few GEFS members. Still, Saturday could go either
way with regards to temperatures. It could be warmer if ECMWF is
right.

We lowered precipitation probabilities significantly for
Sunday/Monday below what was generated by the model consensus
blend that we start with. Models tend to overdue QPF/POPs in these
stable post-frontal environments. 850 mb temperature anomalous are
pretty impressive in EPS and GEFS for Sunday, and temperatures may
be cooler than what we`re currently forecasting.

Mean northwesterly flow aloft and slightly deepening moisture back
to the west on Monday is a favorable pattern for convection to
possibly move off the higher terrain and impact at least western
portions of our area. So, we have low probabilities across the
west Monday night.

BRB

&&

.FIRE WEATHER...
We`ll need to watch Wednesday afternoon for low-end fire
potential. Models have a dry layer that contributes to
dropping of dew points during the afternoon via the mixing process
yielding minimum RH values of 20-25 percent across western
Oklahoma and portions of western north Texas. RH could be lower if
this dry layer is more extensive. This will overlap with winds
around 15 knots, possibly higher. Latest ERC-G observation on the
15th was on the high end of average and dry conditions since may
support values approaching the 90th percentile per forecast progs.
Thus, fuels may be more favorable for initial attack fires than
typically is seen this time of year.

BRB

&&

.PRELIMINARY POINT TEMPS/POPS...
Oklahoma City OK  93  68  93  69 /   0   0   0   0
Hobart OK         95  68  95  69 /   0  10   0  10
Wichita Falls TX  93  68  94  70 /   0   0   0   0
Gage OK           95  70  98  69 /   0  20   0  30
Ponca City OK     92  69  92  69 /   0   0   0   0
Durant OK         93  67  91  71 /   0   0   0  10

&&

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

$$

09/12



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