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Office: FSD

FXUS63 KFSD 172350

Area Forecast Discussion
National Weather Service Sioux Falls SD
650 PM CDT Tue Jul 17 2018

.SHORT TERM...(This evening through Wednesday)
Issued at 427 PM CDT Tue Jul 17 2018

A bit warmer and a creep upward in dew point readings today, but
still a great summer day with a good amount of sunshine outside
areas with just a few more cumulus clouds.  Overnight MCS moving
across Nebraska has left behind a boundary from eastern and north
central NE toward south central SD, and conditions along this
boundary have been gradually destabilizing through the early
afternoon to a point where a few westward drifting showers and
perhaps a rumble or two have popped into existence. Forecast
challenge straddles the shorter and long term periods, primarily
focused around the development and evolution of rainfall on
Wednesday into early Thursday, along with it diagnosis of the
severe weather threat and potential for heavy rainfall.

At this point, the propagation of any storms on the boundary draped
west of the CWA this evening would likely be insufficient to bring
into the area, and have trimmed back evening chances for
thunderstorms toward south central SD.  The latter half of the night
will begin to see a gradual increase in precipitation chances west
of the James River valley as a wave and exit region to strong
digging jet approaching the Yellowstone area around 20z begins to
push out toward southwest and south central SD.  Late night increase
in theta-e transport should increase precipitation chances, even
with the most likely stronger portion of another MCS-type system
drops southeast through NE.

On Wednesday, likely not a lot of intensity with the early day
precipitation as system drops southward toward greater instability
reservoir and leaves a bit of a disturbed wind field toward lower
levels. Will continue to see the mid to upper system start to
increase lift along with a very well-defined linear jet exit region
works toward elongated baroclinicity, especially as the boundary
heads toward mid to late afternoon hours as it stretches out near a
KHON to KFSD to KSUX line. At this point, any more than perhaps an
isolated large hail or wind producing storm during the mid to late
afternoon south of a K9V9 to KYKN line would be fairly surprising,
given the largely slightly more unstable than neutral mid-level
lapse rates, limited surface-based instability, and effective shear
a bit shy of that favoring better organization.  Have shaded temps
down just a bit further south and west due to influx of cloudiness
and expected precip.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 427 PM CDT Tue Jul 17 2018

In terms of the heavy rain threat, this should be evolving during
the very late afternoon into the overnight hours on Wednesday as
the system slows transitioning energy from shear to curvature in
the closing off process. Typical for a cold or transition season
system, precipitation evolution should continue to lift with the
better deformation/theta-e advection more and more to the
north/east ahead of the closed low over northeast SD. The wild
card will be the anticipated slower movement to elements, which
Corfidi vectors indicate around 10kts or less during the
overnight. With the continued progression of the warm advection
wing until system fully closes off, likely not quite as persistent
to get a widespread very heavy rainfall, more on line of a
general inch to inch and a half, but locally higher amounts toward
SW MN and NW IA.

Difference in how system closes off across Minnesota by Thursday are
fairly extreme, with almost 150 miles between the operational runs
in location of closed low. Impact on sensible weather is
significant, either socked in wrap around clouds with temps in the
70s to around 80 per NAM, or the much further northward closed low
in GFS allowing some +32C at at 925 hPa into the Missouri River
along with a greater potential for sunshine over much of the area.
Likely the end results will fall between these two very extreme
prognostications, but shading a bit more toward and EC/NAM evolution
at this point. Still a good chance to shake off much of the
precipitation by midday, with potential for increased instability
developing south of the closed low during the afternoon. If weak
convergence boundary remains aligned across the area, could be a
window for a few severe storms in parts of SW MN/NW IA and nearby
areas during the mid afternoon to early evening hours on Thursday.

A few showers or even a rumble or two of thunder remain possible
into Friday across areas east of I-29 on back side of closed low,
but will generally settle into a bit cooler than normal with
pleasant weather as ridge rebuilds over the area this weekend.
Another frontal boundary looks to arrive around Monday with a shot
for showers/storms as a strong trough along the International Border
works to flatten the ridge and bring another slightly cooler push
after temps early next week.


.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 650 PM CDT Tue Jul 17 2018

VFR conditions will prevail through much of the period, though
scattered 2-4kft cumulus are possible into south-central South
Dakota Wednesday morning. Showers/scattered thunderstorms will
move across the region late tonight through Wednesday, mainly
impacting TAF locations mid to late Wednesday afternoon. Potential
and timing of thunder at specific TAF sites is uncertain at this
time, so will limit mention of thunder to just a couple of hours
at KSUX, though additional TS may be added in later TAF issuances.




SHORT TERM...Chapman
LONG TERM...Chapman

Office: UNR FXUS63 KUNR 172316 AFDUNR Area Forecast Discussion For Western SD and Northeastern WY National Weather Service Rapid City SD 516 PM MDT Tue Jul 17 2018 .DISCUSSION...(This Evening Through Tuesday) Issued at 152 PM MDT Tue Jul 17 2018 A complex series of shortwaves and vorticity maxima are gradually shifting eastward through the northern Rockies and into the northern High Plains this afternoon. The main impulse of interest for our area is crossing the Bighorn Mountains, where isolated thunderstorms have developed. Meanwhile, owing to upslope flow and anomalously high moisture content, isolated thunderstorms are ongoing over the Black Hills. As the rather disorganized vort max moves eastward through MT/WY this afternoon, should continue to see development of convection near and east of the Bighorns. Given CAPE on the order of 1500-2500 J/kg, deep-layer bulk shear of 30-40 kt, and fairly straight-line hodographs, think we could see a mix of supercells and multicells across northeast WY. Through supercell splits, merging outflows (aided by fairly high LCLs), and increased forcing for ascent as the shortwave approaches, should see upscale growth into an MCS as the afternoon goes on. Will also have to monitor the potential for another discrete supercell or two this afternoon over the northern/central Black Hills given localized upslope flow and lengthened low-level hodographs. Primary threat in supercells would be large hail, with straight-line winds becoming the primary threat following upscale growth into an MCS. Borderline 0-3 km bulk shear may limit eastward progression of the MCS into SD overnight, but remaining convection could be reinvigorated as LLJ kicks in after 00z. Flash flooding potential will also be a concern this afternoon through early Wednesday, which is addressed in more detail in the Hydrology section below. The main shortwave shifts into central ND/SD by Wednesday afternoon, which should spell the end of convection for our western zones as subsidence overspreads the area. However, as north/northwestern extent of unstable axis remains over the SD plains, paired with plentiful deep-layer bulk shear, cannot rule out a few strong to severe storms over this area tomorrow. Coverage should overall be limited compared to this afternoon/evening. Fairly high confidence forecast over the second half of the week, with most guidance suggesting a building upper ridge over the eastern Rockies/western High Plains as an elongated surface high noses southward into the Northern Plains from Canada. This should facilitate drier conditions through the remainder of the work week, along with near-normal temperatures. Warmest day should be Saturday as ridge axis settles overhead. Timing of the next trough varies somewhat among model solutions, but generally suspect to see its first impacts by late Saturday or early Sunday. && .AVIATION...(For the 00Z TAFS Through 00Z Wednesday Evening) Issued At 513 PM MDT Tue Jul 17 2018 TSRA with MVFR/local IFR conditions will spread east across the region this evening, ending across northeast WY and the Black Hills by midnight but continuing on the SD plains overnight. Strong to severe storms will be possible. Additionally, heavy rainfall will be possible with any storms. Storms will shift into central SD Wednesday morning. && .HYDROLOGY... Issued at 152 PM MDT Tue Jul 17 2018 Anomalously high PWATs and fairly slow storm movement late this afternoon into the overnight will contribute to locally heavy rainfall over portions of the area, increasing concerns for flash flooding. The Flash Flood Watch has been expanded to include Oglala Lakota County through the overnight period, with a mature MCS expected to cross at least southern portions of the county late tonight. The threat for heavy rainfall will diminish by Wednesday morning as lingering showers and thunderstorms shift southward and eastward. && .UNR WATCHES/WARNINGS/ADVISORIES... SD...Flash Flood Watch until midnight MDT tonight for SDZ024-028. Flash Flood Watch through late tonight for SDZ012-025>027-029- 030-041-042-072-074. WY...Flash Flood Watch until midnight MDT tonight for WYZ054>058-071. && $$ DISCUSSION...Sherburn AVIATION...Johnson HYDROLOGY...Sherburn
Office: ABR FXUS63 KABR 172334 AAA AFDABR Area Forecast Discussion...UPDATED National Weather Service Aberdeen SD 634 PM CDT Tue Jul 17 2018 .UPDATE... Issued at 631 PM CDT Tue Jul 17 2018 Forecast is on track this evening. No major changes are planned at this time. && .SHORT TERM...(This Evening through Wednesday) Issued at 231 PM CDT Tue Jul 17 2018 Forecast challenges surround convection coverage/timing tonight through Wednesday along with impacts cloudiness and areas of precipitation will have on temperatures. Currently, temperatures are warming through the 70s and 80s across the forecast area under a mostly sunny sky and easterly or southeasterly surface winds. A low pressure system is beginning to approach South Dakota now from the west, as well. With the latest suite of model forecasts and guidance, seems more plausible that the northeast corner of SoDak over into wc MN (east of an Aberdeen to Clear Lake line) won't be seeing much in the way of rainfall tonight or Wednesday. As this low pressure moves into the region, thunderstorms, either ongoing or developing, are expected to move across central/south central South Dakota by early Wednesday morning, and follow the best path of mid-level forcing/lift northeast across the forecast area through mid-day. This system is progged to go negative tilt and really slow down (nearly stalling out over the region Wednesday night) while it is over the northern plains region during the next couple of days. This could translate into some rather slow storm motion/movement, potentially increasing a localized excessive heavy rain threat with any thunderstorms around on Wednesday. By the end of the day Wednesday, convection should be clustering over the James River valley of South Dakota, in particular, the forecast zones between the Missouri River and James River valleys. Lots of cloud cover spreading into the forecast area later tonight and during the day on Wednesday. This will likely inhibit daytime high temperatures from realizing their full potential on Wednesday. .LONG TERM...(Wednesday Night through Tuesday) Issued at 231 PM CDT Tue Jul 17 2018 By 00Z Thursday, the northwest to southeast oriented 500mb trough will be set up from northwestern ND through southeastern SD. This feature looks to deepen as it sinks over southern MN during the day Thursday. Taking a look at the sfc features, an area of low pressure looks to quickly slide west to east across SD Thursday morning, becoming better organized across southern MN Thursday afternoon. Still, the potential exists for strong to severe weather Wednesday night. Storms may be ongoing at 00Z Thursday from north central to southeastern SD, sliding to the eastern third of the state after 09Z. The sfc low will slowly exit across northern WI Thursday night, keeping a trough extended across portions of southern SD, before a ridge of Canadian high pressure moves overhead Friday-Saturday. Expect the ridge to exit into MN on Sunday as a lee trough sets up to our west. Forecast models are pretty consistent with the idea of a sfc low across Manitoba by 06Z Monday, dragging a cold front into our forecast area Monday morning to early afternoon. Tuesday should be marked by high pressure filling in behind the Canadian low. && .AVIATION...(For the 00Z TAFS through 00Z Wednesday Evening) Issued at 631 PM CDT Tue Jul 17 2018 VFR conditions are expected at most sites. Showers and thunderstorms will track across the region late tonight and Wednesday. Conditions may fall to MVFR in heavier showers, especially near KPIR. && .ABR WATCHES/WARNINGS/ADVISORIES... SD...None. MN...None. && $$ UPDATE...Wise SHORT TERM...Dorn LONG TERM...KF AVIATION...Wise