FXUS63 KTOP 210448
Area Forecast Discussion
National Weather Service Topeka KS
1148 PM CDT Mon May 20 2019
...Update to aviation forecast discussion...
.SHORT TERM...(This evening through Tuesday)
Issued at 331 PM CDT Mon May 20 2019
No question that the most important forecast challenge for
tonight is just how much rain will fall and where. The severe
weather threat, while non-zero, is clearly greater to the south of
county warning area.
Very strong trough in the desert southwest just starting to turn
to the northeast as wind speed maximum evident in water vapor
imagery ejecting northeast through New Mexico. Evidence of the
strength of the trough can be seen in many different ways. NAM
shows, for example, a substantial mid-level stratospheric
intrusion over central Kansas by Tuesday afternoon with an area in
excess of 10 potential vorticity units down to 300 mb.
With that degree of forcing, a nearly moist adiabatic sounding
for tonight, and little to inhibit convection, rain and
thunderstorms will be widespread and most convective elements will
struggle to become discrete. Effective surface boundary will
likely end up staying to the southeast of the area tonight,
limiting severe potential to areas primarily southeast of
Greatest precip totals tonight are expected to be found along and
southeast of Interstate 35 where an average of 2 to 3 inches
seems most likely, with locally heavier amounts. If surface
boundary is able to progress more northward, higher totals that
NAM is showing might occur, but believe HRRR output is more
realistic. Totals further to the northwest, especially along the
Nebraska border should be the lightest, but will still probably
average 1 to 1.5 inches. Confidence in flooding risk is lower
there, but given antecedent conditions and steady moderate
rainfall occurring there this afternoon, it seems prudent to
maintain flash flood watch for entire area.
Biggest question for Tuesday is probably how much airmass
recovery can take place after overnight/early morning convective
complex rolls through. Deepest moisture will be pushed off to the
east, but very cold mid-level temperatures will lead to steep
lapse rates and even with surface dewpoints in the mid/upper 50s,
moderate instability with CAPE values 1000-1500 J/kg could be
reached. SPC Day 2 outlook has this scenario well covered.
Although QPF amounts will be lighter (and coverage less
widespread) during afternoon, any additional heavy rainfall from
stronger cells could lead to localized flooding risks.
.LONG TERM...(Tuesday night through Monday)
Issued at 331 PM CDT Mon May 20 2019
On Wednesday, broad southwesterly flow aloft will be in place
over much of the Central and Southern Plains with an attendant
surface low located across the High Plains. Models are showing a
strengthening low-level jet Wednesday night which will help to
bring a warm front across the forecast area by Thursday morning.
Thunderstorms are possible along and north of the warm front
Wednesday night into Thursday. MUCAPE near 2000 J/kg and effective
shear ~30 kt could pose a large hail threat.
Throughout the end of the week and into the weekend, midlevel
perturbations within southwesterly flow will traverse the area
with multiple rounds of showers. With that being said, the
potential for flooding is a possibility given the current state of
area rivers and lakes. However, confidence in intensity,
coverage, and timing are limited at this time. Temperature trends
for the long term are expected to remain near normal with highs
near 80 and lows in the 60s.
.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1143 PM CDT Mon May 20 2019
IFR prevails at terminals as light showers with vicinity thunder
to start before the next wave of precip lifts northeast, entering
from west to east in the 9Z to 11Z time frame. Models differ
somewhat on how long precip and IFR stratus lingers into Tuesday
with the latest RAP redeveloping convection through the afternoon.
Meanwhile the HRRR clears to VFR after 18Z so have started the
general trend to VFR by late afternoon as the sfc low exits
northward and strong east winds veer towards the south at that
Flash Flood Watch through Tuesday evening for KSZ008>012-020>024-
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