Area Forecast Discussion
Issued by NWS Bismarck, ND

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FXUS63 KBIS 220235

Area Forecast Discussion
National Weather Service Bismarck ND
935 PM CDT Mon May 21 2018

Issued at 934 PM CDT Mon May 21 2018

Most clouds over the James River Valley have dissipated, while
further west some high clouds are starting to push in from
Montana. Otherwise, quiet weather continues.

UPDATE Issued at 617 PM CDT Mon May 21 2018

For early evening update have increased cloud cover primarily over
the James River Valley and surrounding areas where a cumulus field
has developed. Expect this to dissipate with the loss of daytime
heating. Update has been sent.


.SHORT TERM...(This evening through Tuesday)
Issued at 203 PM CDT Mon May 21 2018

The main forecast issue in the short term period will be
thunderstorm chances Tuesday afternoon.

Latest surface analysis shows surface high pressure over the Great
Lakes region, with a developing lee side trough over the northern
U.S and Canadian Rockies. Southerly return flow has set up over
western and central ND but higher dewpoint air remains to our south.

For tonight, expect mostly clear and quiet conditions, although we
do expect increasing clouds in the west late. We should see warmer
overnight lows, but with light surface winds and clear skies we
still leaned toward the cooler side of guidance for overnight lows.

On Tuesday, the upper low currently over California, tracks slowly
through the inter-mountain west. Lee side cyclogenesis develops over
the central and northern high plains. Ahead of this, moisture return
begins in earnest over western North Dakota. However this coincides
with upper level ridging ahead of the western upper low. Thus the
risk of thunderstorms on Tuesday will be limited to the mid-late
afternoon hours in the far west. Stronger deep layer shear remains
to our south late Tuesday afternoon, but we do become moderately
unstable with MU Cape values around 1500 J/KG over western ND.
There should be sufficient lift for thunderstorms to develop. High
instability will likely lead to a brief window (around 00 UTC
Wed) for a few discrete cells across southwest North Dakota. If
this happens a marginally severe storm is possible with hail
around an inch and gusty winds. The 12z iteration of the 3 km NAM
composite reflectivity field suggests a few discrete cells along
the Montana border around 00 UTC Wednesday.

.LONG TERM...(Tuesday night through Monday)
Issued at 203 PM CDT Mon May 21 2018

Tuesday night through Wednesday morning the upper low center
tracks northward with south-southwesterly flow with multiple
embedded impulses tracking over the high plains. Chances for at
least a few thunderstorms will continue through the night as a
500mb speed max travels over the western Dakotas. Southerly
surface flow continues through Wednesday bringing anomalously high
PWAT values to the state. NAEFS ensemble guidance indicates PWAT
values reaching the 99.5 percentile in areas of western ND with
mean values between 1 and 1.25 inches. Locally heavy rain will be
likely with any convective activity during this period.

Model guidance is consistent with multiple impulses moving early in the day and one in the early evening.
Greatest chances for stronger convective activity remains in the
early evening across the south where steepest mid-level lapse
rates and low level moisture overlap. Bulk layer shear remains on
the low side (20-30 kts) leaving the severe threat marginal for
the area.

The main upper low moves north on Thursday with an attendant
surface trough traveling eastward across the state. Inconsistency
between models and their iterations exist in timing of the trough
axis...leaving still uncertain implications for thunderstorm
threat on Thursday. Either way trends at this time favor the
greatest threat closer to eastern North Dakota.

Progressive flow aloft and associated southerly flow through the
week bring warmer than normal temperatures across western and
central North Dakota. Upper level ridging builds over the Northern
Plains on Friday through the early weekend leaving drier
conditions. Convective activity can`t be ruled out however through
the forecast period given the persistence of low level moisture
and possibilities for upper level impulses through the ridge.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1221 PM CDT Mon May 21 2018

VFR conditions will prevail tonight into Tuesday afternoon. Some
showers and thunderstorms may develop over western North Dakota
Tuesday afternoon.




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