Area Forecast Discussion
Issued by NWS Oklahoma City, OK

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FXUS64 KOUN 190839

Area Forecast Discussion
National Weather Service Norman OK
339 AM CDT Wed Apr 19 2017

Primary forecast focus is placed on thunderstorm chances, beginning
tonight through Friday. Focus on heavy rainfall/flooding and severe
storm potential.

This morning, areas of low clouds are expected to develop/build in
across the region, no fog is expected this morning, as southerly
surface winds will remain elevated through the morning, 10 to 15
mph, increasing after sunrise. Any low clouds will begin to
dissipate and break through the late morning hours. Skies will clear
out west, but partly to mostly cloudy skies will hang on through the
afternoon. Highs will top out in the mid 80s in western Oklahoma to
the upper 70s/lower 80s in central Oklahoma, basically, where clouds
break, temperatures will have the ability to push mid 80s by the mid

Tonight, through the day, a weak 500mb short wave will be traversing
the northern plains, resulting in a frontal boundary setting up and
advancing southward across Kansas through the afternoon into the
evening. A well mixed boundary layer will be in place, expect
surface dew points in the low to mid 60s extending across Oklahoma
into southern Kansas. However, a decent elevated mixed layer will
keep development along the front, in Kansas, at bay until the
evening. The strengthening low level jet, mid-level lapse rates, and
modest deep layer shear, 40 to 50kts, will support large hail
development. Overnight, this activity will take on the features of an
MCS, with 0-3km shear and forward prop. Corfidi vectors suggesting
enough shear cold pool balance to help propagate the cold front
southward through Thursday morning.

Thursday morning, remnant convection from overnight will be ongoing
across portions of eastern and central Oklahoma, with the cold front
slowing, becoming near-stationary along the Red River through the
late afternoon/evening, ahead of the approaching 500mb trough
approaching over the Rockies.

Through the evening and overnight, with the boundary stalling out,
isentropic ascent will increase across the region as the mid-level
wave approaches. In tandem, continued moisture return will result in
precipitable water values anywhere from 1.20 to 1.40 across the
southern plains. Which is well over the 90th percentile from the SPC
sounding climatology. In response, given the broad lift, focus from
the front, and moist atmosphere, excessive rainfall and flooding
concerns continue to increase, especially across central and north
central Oklahoma, where the front will gradually build back north
through early Friday morning, slowly. At the same time, there is a
slight risk of elevated hailers across western and northwestern and
northern Oklahoma, where steepening lapse rates and decent shear and
instability will develop in response to the approaching wave and
increased low level jet. The potential for large, damaging, hail
will be focused over the eastern Texas Panhandle into northwestern
Oklahoma where the influence from the 500mb short wave will be
greatest as it begins to move off the western high plains.

Friday morning, areas of convection will be ongoing across much of
the region. The GFS/ECMWF have come in to better agreement,
synoptically, while the NAM and to a lesser extent the Canadian lag
in the progression of the 500mb trough. Regardless, heavy rainfall
remains a concern through the morning hours across, generally the
northern half of Oklahoma. This is, generally, where the frontal
boundary is expected to reside. Increased precipitation amounts
during the morning hours, primarily north of I-40. where additional
amounts, from the night before, could approach 1 to 2 inches over a
6 hour period, widespread. Of course, given convective properties,
locally heavier amounts could occur, anywhere from 2 to 3 inches or
more. As we move through today and tonight, a Flood Watch may need
to be considered for portions of the region, given overall amounts,
widespread, could range from 1.50 to 5.00 inches in some places.

As for severe storms, there remains considerable uncertainty,
however, with the better agreement between solutions, some of the
details are becoming a bit clearer. I prefer not to re-hash the SPC
Day Three Outlook, it covers the details perfectly. However, here`s
some Cliffs Notes for Friday. Much of how the day evolves will
center around the influence of the ongoing convection through the
morning along and north of the warm front. A lead mid-level wave,
ahead of the dominant short wave will slowly lift eastward through
midday, with a surface low developing over the Texas Panhandle. This
will open up a sufficient warm, moist, southerly flow into a warm
sector positioned over southern Oklahoma into north Texas, with low
to mid 60s dew points pooling south of the warm front and ahead of
the surface trough. MLCAPE values in excess of 2000 J/Kg will
develop with ample westerly mid-level flow and veering profiles
providing more than sufficient shear for updraft growth and
organization. The challenge at hand, that may not truly be resolved
until Friday morning, is how the ongoing convection will influence
the warm frontal placement. Additionally, guidance is quick with the
surface front`s eastward advancement, which, as Joey and Brian
eluded to in the Day 3 discussion, suggests a quick transition to a
more linear mode, with damaging winds becoming the primary hazard.
As details continue to become clearer over the next few days, a more
refined area of where severe storms will be possible will come
together. At present, given the variability in where the warm sector
will set up, severe convection has the greatest potential in the
late afternoon and evening from western north Texas into portions
of southern and perhaps central Oklahoma.

The big takeaways at present: the flooding risk should not be
ignored, especially late Thursday night into Friday across the
northern half of Oklahoma. There is a risk of severe hail
Thursday evening and overnight across portions of western and
northwestern Oklahoma. Severe thunderstorms are likely, Friday,
with the greatest chances in the afternoon into the evening across
the southern half of Oklahoma into north Texas, along the Red
River, but details with respect to exact locations, timing, and
impacts are still a challenge at this time.



Oklahoma City OK  80  64  72  58 /   0  10  30  70
Hobart OK         82  62  73  57 /   0   0  20  60
Wichita Falls TX  82  64  82  62 /   0   0  20  40
Gage OK           85  57  72  51 /   0  20   0  70
Ponca City OK     81  59  71  54 /   0  40  30  70
Durant OK         79  66  80  64 /   0   0  20  20




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