Area Forecast Discussion
Issued by NWS Oklahoma City, OK

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FXUS64 KOUN 120920

Area Forecast Discussion
National Weather Service Norman OK
320 AM CST Sun Nov 12 2017

Little change was needed to the previous forecast. The primary
forecast challenge will continue to be tracking low-level moisture
as it is pushed south by a cold front this morning, but returns
as winds become southerly tonight. Low stratus, drizzle, and fog
will all be possible as moisture returns. Two more cold fronts
will pass during the forecast period--one on Wednesday and the
strongest of all by the end of the week. Weather impacts should
generally be minor, but we`ll need to watch for periods of wind
that could enhance fire concerns.

Latest surface observations early this morning show a cold front
roughly parallel to I-44 moving south. Contrast in air masses is not
particularly significant owing to the relatively low
amplitude/compact progressive nature of parent mid-level shortwave
now approaching the upper Mississippi Valley region. Tricky
forecast this morning with regards to fog and drizzle. Model
forecast soundings are saturated to a sufficient depth in the low
levels for some mention of drizzle, although this should end
fairly quickly as drier air moves in behind the front. The
greatest fog potential may be confined to near and just north of
the front across southwest Oklahoma and western north Texas, and
should be fairly short lived. Dense fog is unlikely. Near term
adjustments to the forecast will probably be needed based on
observational trends.

The cold front will become more diffuse throughout the day to our
south in Texas and surface flow should become more uniform
easterly and then veers/strengthens tonight as westerlies aloft
strengthen over the central/northern Rockies ahead of the next
shortwave. Low level moisture depth will become sufficient late
tonight to support drizzle across a fairly broad area including
nearly all of our Texas counties up through western Oklahoma. We
confined measurable precipitation/PoPs to western north Texas and
far southwest Oklahoma where 0-1km streamlines are most confluent
yielding subtle low level ascent and mean mixing ratio values are
highest. Light/non-measurable drizzle is possible over a wider
area including possibly northwest Oklahoma. There is also some
potential for fog, particularly at the leading edge of moisture
return. Dense fog potential is conditional on how much cooling
occurs early in the night before main moisture surge and if ~3,000
ft stratus layer builds in more quickly. The pattern doesn`t fit
a long duration and/or widespread fog event. As moisture depth
increases and drizzle starts, any particularly low visibilities
should increase.

Moisture will continue to increase on Monday, but with broad low
amplitude ridging influencing the area and lack or large scale
ascent, thermal profiles continue to support just drizzle (i.e., low
level saturation beneath warm/dry layer aloft). So, we will continue
to emphasize "drizzle" wording in the public forecast. Similar to
Friday, drizzle may persist beyond the climatologically favored time
of day, and continue through the afternoon given limited
opportunity for diabatic surface heating and mixing layer growth
and resultant shrinking cloud layer. We were quite liberal with
coverage of drizzle through the day and into the night.

On Tuesday, 850 mb flow veers to more southwesterly and a
drier/warmer pattern is expected. Many locations, especially
across the western half of the area, should rise above 70 degrees
for the afternoon high temperature.

As mentioned above, a cold front is expected to move through on
Wednesday tied to yet another transient wave to our north.
Thermal/moisture profiles look a little more favorable for some
more organized precipitation (i.e., showers). Although low-level
saturation is still fairly extensive in model forecast soundings
beneath a warm layer. Most of the ascent will remain north of us
so it`s unlikely that the warm layer will be lifted out, so
confidence in precipitation is low. Perhaps we can at least get yet
another brief period of drizzle near the front.

Not much cold air is expected behind this front either. In fact,
until the pattern becomes more amplified and less progressive
we`re not likely to see a significant cold surge into the central
and southern Plains. Southerly flow is expected to return
Thursday in response to another (stronger) trough. This is where
models diverge significantly. GEFS members are fairly close to the
mean, although there are a couple that lag behind more similar to
what ECMWF has persistently shown. Also, several are deeper, more
amplified and closed off. The Canadian is a compromise in timing.
Bottom line, a more significant system with a stronger cold front
bringing a period of warmth and wind, followed by a modest cool
down, is expected. The frontal passage will quickly occur sometime
between Friday morning and Friday evening depending on which
model you believe. Any substantial/organized precipitation should
be limited to higher latitudes near stronger forcing for ascent.



Warmer temperatures and 20-25 mph southwesterly winds with gusts
over 30 are expected Tuesday across portions of the area, mainly
western Oklahoma. Low level moisture should be extensive enough to
preclude significant fire concerns. Minimum relative humidity
values where the area of strongest winds are expected should stay
above 50 percent.

The next period of potential fire weather will be Friday as a
progressive strong trough passes to the north. These type systems
generally support more of a consolidated Pacific frontal passage
rather than a preceeding defined dryline, so relative humidity
should stay fairly high ahead of the front. Such systems can cause
substantial pressure rises immediately behind the front
supporting strong winds. Our public forecast and fire weather
planning forecast doesn`t yet reflect this possibility since we
used a blend of medium range models as a compromise between
significant differences in timing. As we approach Friday, we`ll be
able to refine frontal timing and depending on such factors as
timing with relation to the diurnal cycle, some enhancement of
fire potential may occur, particularly behind the front.

With regards to fuels, recent freezes have brought us into the
dormancy period. Latest ERC-G observation from the 8th suggests
that values have come down closer to climatology, and the trend
was indicated to be downward, probably because of a prolonged
period of drizzle albeit light and high RH. Thus, fuels
combustibility does not appear anomalous through Friday.



Oklahoma City OK  57  44  58  55 /  10  10  10  20
Hobart OK         55  45  58  54 /  10  10  30  20
Wichita Falls TX  61  50  62  57 /  20  10  20  10
Gage OK           55  40  58  50 /   0  10  10  10
Ponca City OK     56  41  57  53 /   0  10  10  20
Durant OK         62  51  64  54 /  20  10  10  10




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