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Area Forecast Discussion (AFD)

Issued by Norman, OK (OUN)

                            
138 
FXUS64 KOUN 250931
AFDOUN

Area Forecast Discussion
National Weather Service Norman OK
431 AM CDT Mon Sep 25 2017

.DISCUSSION...
No significant changes were needed to the previous forecast. The
primary forecast challenge over the next several days will be
timing of several periods of showers that are expected to impact 
the area. 

Latest water vapor imagery shows deep closed low now accelerating
northeastward across Wyoming and a fairly substantial plume of 
mid-level dry air advecting across the Panhandles into Kansas. 
One vort max/PV anomaly is now rounding the base of the trough 
near the Four Corners region and will enhance ascent across much 
of the central and southern Plains today as it quickly moves
through. Already, convection has increased across the New Mexico 
Plains. Further east, isolated/scattered convection has increased 
within strengthening low level jet/warm conveyor. Convection has
been most numerous across Kansas where warm conveyor is lifted
atop a cold front that is slowly advancing southward. This cold
front is expected to continue southward into our area late today 
and slowly moving south across the entire area by late tomorrow. 

We have retained relatively high precipitation probabilities today
within the belt of deep/anomalous moisture (PWAT values
1.50-1.75). Even subtle ascent in such a moist environment
generally yields decent convective coverage. Instability will be
weak so thunderstorm coverage will be limited, and severe
thunderstorms are not expected. Current thinking is that a steady
uptick in convective coverage will occur through the morning with
a maximum occurring early-mid afternoon before ascent rapidly
shifts northeastward out of the area and mid-level height rises 
commence.

Confidence in coverage later tonight into Tuesday is in
question given building mid-level ridging and weakening warm 
conveyor. Some of our more widespread precipitation events 
necessitating categorical probabilities typically occur in the 
vicinity of fronts when orthogonal flow is fairly substantial, 
similar to what's happening in Kansas tonight. But there is some 
doubt if comparatively weak flow over the cold front will be 
substantial enough to support the high end probabilities we 
currently have. Nevertheless, with such anomalous moisture (PWATs 
rising to 1.75-2.00) we have kept probabilities fairly high 
tonight into Tuesday. We have trended them down only slightly but 
we may need bring them down further in subsequent forecasts.

Tuesday night, modest flow atop frontal surface should result in
an increase in convective coverage, particularly across our Texas
counties into southwest Oklahoma. This should continue to spread
northward across northern Oklahoma during the day Wednesday. This
is in response to deep closed low in the Southwest that will move
slowly northeast enhancing isentropic ascent in a continued 
anomalously moist environment. Eastward extent is in question but 
best pressure on 305-315K surfaces in the models is roughly west 
of Highway 81.

Our latest storm total rainfall amount forecast has 2" or greater
across roughly the western third of the area with considerably
lower amounts further east. We have up to about 4" in far
southwest Oklahoma but this is conditional on the above described
scenario for Tuesday night and Wednesday materializing. Flooding 
seems unlikely for a couple of reasons. 1.) duration of rainfall 
(several rounds over a few days), 2.) limited instability not 
supporting robust convection thus limiting particularly high 
rainfall rates with individual convective elements. 

We have kept chances low for the latter portion of the week into
the weekend. Closed Southwest low will weaken and move northeast
by then. GFS and ECMWF are completely out of phase by Sunday into
early next week leading to a low confidence forecast. 

A much cooler pattern is expected with this frontal passage. Below
normal temperatures are expected. Our temperature forecast may not
be cool enough where rainfall persists the longest. Portions of
the area (particularly northwest Oklahoma) could struggle to rise
out of the lower 60s on Tuesday and Wednesday. 

BRB

&&

.PRELIMINARY POINT TEMPS/POPS...
Oklahoma City OK  86  68  72  62 /  30  50  70  30 
Hobart OK         78  63  68  59 /  70  80  70  40 
Wichita Falls TX  85  69  77  64 /  50  60  70  60 
Gage OK           75  54  64  54 /  80  70  50  20 
Ponca City OK     87  66  71  60 /  40  80  70  30 
Durant OK         87  68  88  68 /  10  10  30  40 

&&

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

$$

03/12