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Area Forecast Discussion (AFD)

Issued by Topeka, KS (TOP)

FXUS63 KTOP 162046

Area Forecast Discussion
National Weather Service Topeka KS
346 PM CDT Tue Apr 16 2019

.SHORT TERM...(This evening through Wednesday)
Issued at 340 PM CDT Tue Apr 16 2019

A severe weather threat remains on the docket for Wednesday
afternoon, with large hail, damaging winds, and heavy rainfall the
main threats.

Several forecast challenges lead up to the severe weather event 
for Wednesday afternoon. The first couple of hurdles that need to 
be jumped are with respect to cloud cover, precipitation chances,
and temperatures. Lower-tropospheric moisture gradually streams 
northward on a broad southerly fetch through Wednesday afternoon. 
Increasing 300K isentropic lift and theta-e advection under a 
strong EML may lead to the development of a thin 
stratus/stratocumulus deck tonight. The early signs of this cloud
deck have been noted in satellite/surface observations in 
Oklahoma and the GFS/RAP support these clouds advecting into the 
forecast area overnight. There is some discrepancy in the 
thickness of the cloud shield, with the RAP depicting a very thin 
deck as opposed to the habitually over-saturated NAM solution. For
the going forecast, have trended closer to the RAP in the timing,
height, and behavior of the clouds.

The first hurdle overcome, we now turn to precipitation chances 
for tonight. The aforementioned EML and shallow nature of the
stratus will substantially limit the threat for precipitation and
have trended drier in the going forecast. There are some signals
in the RAP bufr soundings of a weakening in the inhibition closer
to the lifting surface boundary/warm front along the Nebraska 
border and have maintained the mention of a stray shower or 
thunderstorm in this area. Earlier CAM runs were quite bullish in 
the areal extend of their PoP and QPF fields, but a closer 
inspection of these fields, in combination with forecast 
soundings, points towards more convective feedback noise driving 
these values than realistic meteorological solutions. The synoptic
models trended back on PoPs 12 hours ago and only now are the 
HRRR/RAP likewise backing off on these chances. Did keep some 
mention of sprinkles in the forecast along and north of I-70 to 
account for any stray, shallow showers that manage to generate 
along the best forcing axis attendant with a passing H500 wave, 
but these should have minimal impacts on the sensible weather.

Highs for Wednesday will depend on how quickly the low clouds mix
out, but given the thin nature of the clouds, am expecting that
clearing should take place by mid to late morning with a rapid
warming trend into the afternoon. Increased highs into the low 80s
in agreement with the GFS/EC/CMC guidance given the strong warm
nose in place and ample mixing ahead of the approaching front.

We finally turn our attention to Wednesday afternoon's severe
weather threat. The overall timing, coverage, and mode forecast
has varied little from previous updates. With the northern and 
southern stream waves remaining slightly out of phase until 
Wednesday night, the threat for the best severe weather appears 
tied to the subtropical low over TX and OK. Across the forecast 
area, southwesterly synoptic flow will feature a large line- 
normal component relative to the orientation of the cold front and
rapid upscale growth into line segments and clusters is expected
shortly after convective initiation in the mid to late afternoon
hours along and east of a line from Abilene to Marysville.
Forecast hodographs also feature multiple inflection points and 
loops that would not favor discrete cells. These storms will move 
in a northeasterly fashion with the entire system shifting 
eastward into the evening. MLCAPE values of 1500-2000 J/kg and 
deep shear of 30 to 40 kts would support any robust updrafts that 
manage to become sustained. Large hail--possibly up to 2 inches in
diameter--and strong winds would be the main threat. Any line 
segments that roll up the surface boundary could become oriented
into a favorable QLCS tornado alignment, but with 0-3 km shear 
values of only 15 kts, it will take a large mesoscale augmentation
of the pattern to realize this potential. The heavy rain threat 
would likely manifest itself after 00Z and is covered in the long-
term discussion below.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 340 PM CDT Tue Apr 16 2019

Storms persist Wednesday night across east-central Kansas, with 
temperatures briefly cooling down before rebounding by the 

Convective coverage along the surface to H850 boundary should be 
widespread throughout Wednesday night owing to strong forcing for 
ascent and unidirectional, boundary-parallel storm-relative flow.
The near-surface front advances slowly, but steadily, 
southeastward during the night and should clear the forecast area 
around sunrise Thursday. There is some concern that a flash flood 
threat may develop, mainly along and southeast of the Kansas 
Turnpike, during the overnight hours as multiple waves of 
convection ride up and along the lower tropospheric baroclinic 
zone. Model progs depict strong low-level moisture convergence and
H925-700 mixing ratios of nearly 10 g/kg. PWATS of 1.25"+ are 
well above the 90th percentile of values observed on the TOP/SGF 
RAOB for this time of year, possibly even approaching the daily 
max value of around 1.4". Thankfully, the pattern has been dry 
going into this event and flooding potential may hinge more on 
rainfall rates than amounts (at least initially). Multiple 
convective resolving and parameterizing solutions print 1 to 2+ 
inches of storm-total QPF over the southeastern forecast area and 
given the meteorological setup, these solutions are within the 
realm of possibilities.

Thursday looks to be blustery and cooler with highs topping out 
in the low 60s and northwesterly winds of 10 to 15 kts--with steep
low-level lapse rates mixing down gusts of 25 to 30 kts. The 
surface cyclone and attendant deformation zone should reach the 
Great Lakes region Thursday afternoon, therefore the chance for 
any showers Thursday afternoon will be dependent on the degree of 
solar insolation and differential CAA during the peak heating 
hours. With this latest update, have removed most of the 
precipitation chances across all but the far northwestern forecast
area during the afternoon where deeper convective processes could
be realized.

Large scale subsidence should lead to quiet weather for Friday 
and Saturday with the surface ridge axis passing through Friday 
afternoon. As WAA ensues, highs rebound back into the upper 60s 
on Friday and mid-70s for Saturday under mostly clear to clear 
skies. The next chance for precipitation comes late in the weekend
as a series of subtle upper-tropospheric shortwaves propagate 
around the periphery of a western North America longwave trough. 
These waves will interact with a surface baroclinic zone 
meandering southward through the region, but it is too soon to pin
down the exact timing of this precipitation. Early next week does
look to feature cooler temperatures behind this surface trough, 
but given the subtleties in this pattern and inconsistent inter-
and intra-model continuity, confidence is low in this forecast. 


.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1240 PM CDT Tue Apr 16 2019

VFR conditions are expected for the rest of the afternoon and into
the evening, with MVFR stratus possibly developing after 06Z
across mainly the eastern two-thirds of the forecast area. A few
light showers and a stray thunderstorm are possible, mainly north
and east of the TAF sites. South to southwest winds persist for
the period, with gusty winds today decreasing and LLWS developing
overnight, with gusty winds setting in again Wednesday morning.