Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus64 koun 210929 
afdoun

Area forecast discussion
National Weather Service Norman OK
329 am CST Thu Nov 21 2019

Discussion...
the main focus for this forecast is to time out precipitation
chances, determine precipitation type later tonight in northern
Oklahoma, and to deal with below average predictability early
next week.

A few factors could lead to the development of a narrow band of
shallow convection early this morning. 1) mechanical lifting
along the front moving southeast, 2) passage of low amplitude
shortwave ridging aloft and subsequent onset of midlevel height
falls from approaching trough, 3) increasingly deep
moist/saturated layer in the low levels, 4) around 300-500 j/kg
MUCAPE distributed thinly through a relatively shallow layer.
Some high-res guidance shows locally enhanced quantitative precipitation forecast amounts of
0.50-1.00 inch, and that seems reasonable in the most productive
scenario with regards to convective development. This initial
development could occur in the 6am-8am timeframe near and just
south of the I-44 corridor given observation data and expected
frontal location.

Thermodynamic profiles south of advancing front later in the
morning are not expected to be much different than the above
described, but slightly higher instability by midday or early
afternoon with diurnal heating is possible. Modest instability
and strong deep layer shear could result in a few strong to severe
storms in the far southeast portion of our area (the Atoka,
Durant, Lake texoma area). Similar limitations to yesterday exist
which should preclude higher chances of strong to severe storms.
Midlevel lapse rates are not very steep and hail growth zone cape
is weak. Downward momentum Transfer in heavier precipitation cores
could cause some strong gusts, but severe gusts should be
tempered by overall lack of convective vigor and nearly non-
existent dry sub-cloud layer. Veering/strengthening low-level
flow could contribute to transient supercellular characteristics
within any shallow individual cells that form.

Later in the day, deep moist ascent on 290-300-k isentropic
surfaces will support rain development. We kept wording "showers"
since some elevated instability is shown in forecast soundings and
this may lead to quasi-cellular characteristic. A few weak
thunderstorms may be possible too, but the better chance of storms
will be close to the Red River as described above. Highest
precipitation probabilities were reserved for the stronger forcing
for ascent tied to maximized DCVA as the trough moves in later
this evening. A slight slowing/deepening trend was noted in the
models with this trough, and so we have slightly higher
precipitation chances mainly across northern Oklahoma Friday
morning. Overall, this shouldn't change the forecast drastically.

There is still some uncertainty on precipitation type in northern
Oklahoma late tonight/early Friday. There could be a loss of ice
introduction due to drying of mid levels during the morning and a
period of light drizzle/freezing drizzle could occur in some
areas even after rain had already changed to snow. Overall, light
precipitation amounts and temperatures right around freezing
should significantly limit or preclude impacts. Light accumulation
amounts of snow appear possible in northwest Oklahoma, but
generally less than a half inch as it appears based on current
data.

The next system could impact the area by Tuesday. Medium range
deterministic and ensemble guidance is starting to converge on a
more open and less amplified system, in contrast to a deep/intense
system shown in a few runs over the past few days. Assuming this
trend sticks, the pattern may support a more substantial, albeit
brief, cold surge than currently indicated in the forecast. Any
precipitation should be light since warm/moist conveyer will be
east of US and our atmosphere will be quite dry. We'll watch
trends closely as a more impactful scenario is still possible,
especially if the trough amplifies further than most guidance
currently indicates and the surface low deepens near/northeast of
US (this is shown in some ensemble members). This would at least
result in windy conditions for US, and perhaps a more favorable
moisture scenario via trowal/deformation development near our
area. This is a low probability scenario though and only low
probabilities of precipitation are appropriate for this period
given current data.

Brb



&&

Preliminary point temps/pops...
Oklahoma City OK 55 37 46 30 / 60 80 30 10
Hobart OK 55 34 47 28 / 30 90 10 0
Wichita Falls Texas 63 40 52 32 / 60 90 20 0
gage OK 48 29 43 26 / 20 70 20 0
Ponca City OK 55 34 43 26 / 20 60 40 10
Durant OK 71 44 54 33 / 80 90 50 10

&&

Oun watches/warnings/advisories...
OK...none.
Texas...none.

National Weather Service Glossary of Abbreviations