Scientific Forecaster Discussion
fxus64 koun 261108
Area forecast discussion
National Weather Service Norman OK
608 am CDT Tue Sep 26 2017
for the 12z tafs:
Poor flying conditions will be experienced across much of the area
today. Closer to a stationary front extending from eastern
Oklahoma to near the Red River down into western North Texas,
showers are expected to be most numerous. MVFR to IFR ceilings
will be common near and north of the front as well. Ceilings are
most likely to be at least temporarily VFR further north across
northern Oklahoma. Periods of showers could result in visibility
reductions, possibly to IFR in heavier showers. There may be a
relative minimum in activity sometime this afternoon followed by
more widespread showers affecting at least southwest taf sites
later this evening and through the night.
Previous discussion... /issued 420 am CDT Tue Sep 26 2017/
periods of rain showers are expected through at least midday
Wednesday. These anomalously moist environments with relatively
subtle forcing can be difficult to forecast timing of highest rain
chances. This remains the primary forecast challenge.
Early this morning, a cold front extended from just east of okc to
near Wichita Falls and into the Low Rolling Plains of West Texas.
Convection has gradually increased across the area over the last
several hours. The greatest coverage has been from central
Oklahoma into North Texas where deeper moisture exists and flow
atop the boundary is slightly stronger (i.E., Stronger isentropic
ascent). A cluster of convection has been progressing east-
northeast across lubbock's area tonight, near the front and along
the low-level Theta-E gradient. There has been a tendency for
slight rightward propagation into higher Theta-E.
We have shifted the band of highest precipitation probabilities
for today southward, but possibly not far enough south. Although
several locations in the northern half of Oklahoma will still
probably see measurable precipitation, the greatest concentration
should be closer to the front through the morning. Through the
day, convective may trend downward markedly. There is some
uncertainty in this. Despite somewhat weaker flow atop the
boundary as the day progresses and northern stream wave departs,
nearly stationary surface boundary to our south and anomalous
moisture (1.75"+ precipitable water which is near the climatological max) should
support at least some convection. Models suggest upper ridge
amplification and strengthening of the upper level jet just to our
northwest. This may be augmented somewhat by remnant latent
heat/high level moisture plume from remnant T.D. Pilar in Mexico.
This setup justifies keeping at least high chance if not likely
probabilities for the latter portion of the day. We may need to
adjust this based on trends, however.
Attention then shifts to a second system in the southwest.
Evidence of Pacific energy moving south through California is seen
now in water vapor imagery, and this will will carve out a closed
low today in the southwest. Low level mass response across our
region will be sufficient for strengthening isentropic
ascent/saturation and precipitation generation tonight, mainly
focused across our southwest counties. This will shift northward
across western Oklahoma through at least the first part of the
day. Eastward extent of the most widespread convection is somewhat
in question so refinement of the forecast for this period may be
needed later today.
Instability will be very weak at best the entire time and
thunderstorms will have far less coverage than showers.
Thunderstorms that do form will be quite weak and severe storms
are not expected.
Deep warm cloud depth and anomalous deep moisture may support some
fairly high rainfall rates. Limited instability and thereby
limited convective intensity is one of the main limiting factors
for flooding potential. A relatively long duration 2-3 inch rain
event (one round this morning, and another tonight/tomorrow
morning) with locally higher amounts is expected across the
southwest portion of the area. Some flooding is possible and so we
will continue the Flood Watch for five of our western North Texas
counties, although the above mentioned limitations should limit a
more significant flood concern.
From Wednesday night Onward, some lingering showers may occur in
the southwest portion of our area but coverage should be minimal
at best. Medium range models are still somewhat out of phase
toward the end of the forecast period resulting in below average
confidence for the day 5-7 period of the forecast. Aside from a
small chance of convection in far northwest Oklahoma late Sunday,
we've kept the extended period dry. The general consensus of
medium range guidance is for a warmup to near climo by the latter
half of the weekend, to slightly above climo (low-mid 80s common
across the area) by early next week.
Preliminary point temps/pops...
Oklahoma City OK 66 60 67 58 / 80 60 70 20
Hobart OK 64 59 65 57 / 80 80 80 30
Wichita Falls Texas 73 63 70 60 / 90 80 70 30
gage OK 63 54 64 53 / 40 30 50 20
Ponca City OK 68 60 69 56 / 70 50 60 10
Durant OK 85 67 82 64 / 60 40 30 20
Texas...Flood Watch through Wednesday evening for txz083>085-087-088.