Area Forecast Discussion
Issued by NWS Oklahoma City, OK

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FXUS64 KOUN 021138

Area Forecast Discussion
National Weather Service Norman OK
538 AM CST Fri Dec 2 2016



.AVIATION... /For the 12z TAFs/
VFR conditions will continue through the day. High cirrus clouds
will overspread the entire area and gradually lower and thicken.
MVFR ceilings and rain should hold off at all sites until around
00z (starting in the southwest) and spreading northeast through
the end of the TAF period. This is a high confidence rain event
and visibility and ceilings will gradually lower through the


.PREV DISCUSSION... /issued 407 AM CST Fri Dec 2 2016/

Latest water vapor and IR satellite imagery shows cirrus
increasing to our west ahead of a deepening trough in the
Southwest United States. Models are in agreement that this trough
will dig southward and evolve into a deep closed low that moves
into northern Mexico by tomorrow night. The upper flow pattern
will amplify as this occurs. Strengthening jet level momentum across
the Central Plains extending into the Midwest will result in a
broad area of upper level divergence and deep ascent across a
large portion of our region. High level saturation and thick
cirrus during the day tomorrow will be a precursor to gradual
moistening/saturation at lower levels.

By late tomorrow deep saturation is shown in forecast soundings
across the southwest portion of our forecast area. This will be
coincident with ascent throughout a deep layer as is seen on
isentropic analysis. This signifies a straight-forward rain event
with high confidence. We have increased precipitation
probabilities to categorical across the southwest portion of the
area tonight and have spread these high probabilities eastward
across southern Oklahoma tomorrow.

We have continued with a preference for raw model guidance with
regards to temperatures tonight and tomorrow. Model forecast
soundings suggest more pronounced warm advection in thermal
profiles than what had been shown previously. Temperatures will
be warm enough for all rain except for possibly the far northwest
portion of Oklahoma. NAM and GFS Bufkit soundings at Woodward show
>1,000 ft of above freezing wet bulb temperatures, with a warming
layer just above that which is nearly isothermal and slightly
warmer than freezing. Given the marginality, especially just
northwest of the Woodward, we have kept comparatively lower
probabilities of snow in the forecast and kept rain probabilities
higher. This is generally from around Gage and Woodward and
extending northwestward, including most of Harper County.
Impactful wintry precipitation is not expected.

As entrance region of upper level jet max moves further away by
Saturday evening, ascent will wane and we have decreased
precipitation probabilities accordingly. Sunday should be dry for
most locations except across the south where weak ascent will be
present ahead of the opening/ejecting mid-upper trough. As this
occurs, a second round of precipitation should spread northward
across mainly the southeast half of the area Sunday night through
Monday. Instability will be weak at best but across the southeast
portion of the area a few hundred joules of MUCAPE from elevated
parcels may support some thunder, but will be insufficient for
strong/severe storms.

A much colder pattern is anticipated as a broad trough and
associated surface cyclone dislodge cold arctic air mass sending a
strong cold front southward through the area by the middle of the
week. Exact timing is unclear given discrepancies between medium
range deterministic guidance and spread among medium range
ensembles. The onset of the coldest air should be sometime
Wednesday. Model disagreements grow toward the end of the week but
there is some signal that significant moderating of temperatures
may occur as soon as Friday. Our current forecast represents a
compromise between the guidance that shows a quicker return of
warmer air and model guidance that lingers the cold air mass.

There are two other forecast considerations in the long term. 1.)
Potential for a period of strong winds behind the cold front. 2.)
Light precipitation chances Tuesday night and Wednesday.

We decided not to deviate from the blend which averages out winds
since both GFS and ECMWF are spread about 12 hours with when the
strongest post-frontal surface pressure rises move into the area
sometime Wednesday. We will likely need to significantly increase
the winds once timing details are worked out.

Low to mid level saturation is show in model forecast soundings
Tuesday night into Wednesday as weak ascent spreads across mainly
the northern portion of the area with the next trough. Thermal
profiles support mostly light snow during this period.



Oklahoma City OK  58  42  46  40 /  20  70  70  50
Hobart OK         55  41  45  40 /  40  80  70  40
Wichita Falls TX  59  45  47  42 /  50  80  80  50
Gage OK           52  35  41  34 /  20  70  50  30
Ponca City OK     56  39  47  39 /   0  40  50  40
Durant OK         61  45  46  43 /  20  70  80  70




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