Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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458
FXUS63 KFSD 301125
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
625 AM CDT Sun Apr 30 2017

.SHORT TERM...(Today and Tonight)
Issued at 353 AM CDT Sun Apr 30 2017

Primary story in the short term is with rain and then a very unusual
and perhaps historic May 1st snowfall. We`ll start out today`s
discussion with a few observations within tonight`s forecast.

1.) Model guidance has again overestimated QPF ahead of this system
tonight and this will continue into the morning hours. Partially due
to an interrupted moisture flow from convection over the Ozarks, but
also due to the lack of very strong forcing and a very dry airmass
residing and being reinforced this morning.

2.) The digging system has slowed down considerably over the past 24
hours, and with precipitation remaining light, overnight
temperatures may struggle to fall below 40. This also means that
temperatures will again climb warmer during the day, especially
northwest of a Yankton to Sioux Falls to Marshall line. Have
considerably bumped up highs in most areas.

3.) Looking across western Kansas and southern Nebraska for
precipitation type analogs for us later tonight, seems to suggest a
mix begins around 36 degrees with full transition to snow by 34.
This may at least allow us to narrow down the area of snow once this
system pivots northward.

4.) Snow will likely be dictated by both rate and boundary layer
temperature. Strong dynamic cooling and forced boundary layer
temperature cooling will only be found in the moderate precipitation
rates. Elsewhere tonight, there have been pockets of 40 degree
readings.

5.) The recent westward precipitation trend in the trowal has
shifted slightly back to the east in the past 12 hours. Based on
inclusion of high-res guidance, this will leave the far western CWA
potentially out of the precipitation all together and an extreme PoP
gradient is likely further west.

Onto the sensible weather, have generally blended GFS/EC with
influence from the HRRR/RAP for the first 12 hours. Very light rains
will slowly spread northward this morning, and without any strong
influence of moderate precipitation, we may see temperatures climb
into the lower or even mid 40s.  Huron to Chamberlain could even see
mid-50s. Strong forcing begins to move in by this afternoon and
evening, with convection potentially forming over eastern Nebraska
and western Kansas. Have not included thunder, but it`s certainly
possible from Sioux City to Spencer later this afternoon.

One bigger change from the previous forecast was to slow the cooling
through the late afternoon and evening hours. Most models
consistently keeping temperatures from 38 to 41 within the
precipitation axis at 00z. Thoughts are that as more intense
precipitation arrives later in the evening temperatures will be
forced cooler and towards the 34 to 37 range in a narrow corridor.
It`s likely at this point that we begin to see pockets of snow form
as dynamic cooling is maximized. This will certainly be maximized in
the higher terrain areas first, then slowly spreading across the
heart of the CWA as the narrowing precipitation corridor begins.

Snow will likely persist into mid-morning Monday, especially over
southwest Minnesota and the I-29 corridor. As far as accumulations
are concerned, after coordination with WPC and running various SLR
blends, have backed off on the SLR towards an 8-9:1 ratio after 00z.
 Some accumulation mitigating factors include rate, warm ground
temperatures, and wet ground from antecedent rainfall. Nevertheless,
likely to see a corridor of 1" to 4" across a narrow area of the
CWA, with some potential for localized enhancement of up to 6 or
more inches in the Buffalo Ridge. This event has the potential to be
a boom or bust type of event with critical boundary layer
temperatures key to little or even much higher snow amounts, so
mesoscale monitoring will be key.

Through coordination, will issue a winter weather advisory along the
most likely corridor or snow, beginning at 03z and ending by noon
Monday.  This will allow the dayshift to monitor the higher terrain
areas and upgrade to a warning if needed. While 1 to 3" of snow will
be short lived and typically don`t lead to high impacts, it`s the
unusual nature of the event, and the impact to Monday morning
commute that owe to issuing the advisory.

Behind this system, we should clear out sharply by the afternoon
hours over central SD. In fact, deep mixing and brisk winds may push
highs well into the 60s west of the James River.

.LONG TERM...(Monday through Saturday)
Issued at 353 AM CDT Sun Apr 30 2017

After the excitement and perhaps record breaking of the short term,
the mid and long range portions of the forecast will be tame by
comparison.  Impacts from the large storm will be evacuating fairly
quickly Monday night, with perhaps a couple of lingering showers
rotating through southwest MN to near the Iowa Great Lakes, but
generally looking for the mixing-driven winds to start to drop off
fairly quickly by late evening. Areas with snow cover could drop
off to around 30 degrees with wind keeping temps up a bit more,

While the primary closed low will shift into the Great Lakes on
Tuesday, a couple of trailing waves will settle southeast into the
Northern Plains through midweek. Tuesday afternoon could find a
couple of showers encroaching on areas near/west of the James River
valley during the afternoon.  After settling down during the night,
a stronger jet streak dives into the region on Wednesday with a
little more significant convergence zone. Timing with the diurnal
cycle, may have to watch for some thunder within the greater shower
pattern, which may need some boosting in coverage/amounts.

Temperatures Tuesday will have to fight the residual snow cover, and
have trimmed a bit along the expected snowfall axis. Temps climb up
to near normal on Wednesday, but no widespread frost/freeze threats
in wake of storm.

The latter portions of the forecast from Thursday through Saturday
look to be dry, as a ridge of high pressure aloft begins to expand
eastward into the Plains by the weekend.  The 00Z ECMWF run does try
to hold in some less mild air through Friday with more persistence
to the cooler surface ridging, but the brakes are taken off the
temps in a vast majority of solutions by the weekend, when 70s
should become much more common for highs.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 622 AM CDT Sun Apr 30 2017

Linear band of rain slowly trying to move northward into dry air.
It may reach Sioux Falls and then slow down on it`s northward
track. Generally very light precipitation expected at FSD/SUX
through mid-day. Rainfall rates should increase by mid-day and
through the afternoon. Ceilings will fall into the MVFR to IFR
category through the day, remaining at IFR levels into the night.

Have significantly delayed arrival of any snow to the terminals
tonight. Regardless, Sioux Falls stands the highest risk of
accumulation snow, should temperatures cooperate.

At Huron, rain may stay just southeast of the terminal, so have
not introduced any precipitation or lower ceilings in this taf
issuance.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Winter Weather Advisory from 10 PM this evening to noon CDT
     Monday for SDZ039-040-054>056.

     Winter Weather Advisory from 10 PM this evening to 9 AM CDT
     Monday for SDZ060>062-065-066-068-069.

MN...Winter Weather Advisory from 10 PM this evening to noon CDT
     Monday for MNZ071-072-080-097.

IA...None.
NE...None.
&&

$$

SHORT TERM...Dux
LONG TERM...Chapman
AVIATION...Dux



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