Area Forecast Discussion
Issued by NWS Bismarck, ND

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FXUS63 KBIS 281151

Area Forecast Discussion
National Weather Service Bismarck ND
651 AM CDT Fri Apr 28 2017

Issued at 651 AM CDT Fri Apr 28 2017

No significant changes were needed with this update. A surface
ridge is maintaining its influence across the area, with light
winds and a cold start to what is still on track to be a warmer
day than yesterday.


.SHORT TERM...(Today and tonight)
Issued at 334 AM CDT Fri Apr 28 2017

A warming trend will continue today along with light winds. Highs
are forecast to be in the mid to upper 50s F in most areas. Very
dry air will also be in place.

Weak shortwave ridging at 500 mb is expected across western and
central ND today downstream of a northern-stream shortwave trough
moving into western Manitoba, and north of a deep trough digging
into the Four-Corners region of the southwestern U.S. The 00 UTC
April 28th RAOBs sampled a very dry air mass across the northern
plains characterized by impressively-low precipitable water values
near 0.20" at both Bismarck and Aberdeen. Those values are very
close to the daily minimum values ever observed at those sounding
sites in late April. The subsidence-dominated regime today will
likely foster additional drying through the column, and indeed the
00 UTC NAM and GFS both suggest precipitable water values falling
a bit more to around 0.15" across central ND by this afternoon.

Despite lackluster mechanical mixing Thursday owing to weak low-
level wind fields and an inversion based near 775 mb per the 00
UTC Bismarck sounding, very steep lapse rates in the boundary
layer, the exceptionally dry air mass and full sunshine produced
strong diabatic warming at the surface. Dewpoints also dropped to
around 10 F in northwest ND in the core of the lowest deep-layer
moisture content. Forecast soundings valid this afternoon reveal
very similar characteristics and even suggest the boundary layer
may be somewhat deeper compared to the one observed Thursday. The
warmer edge of guidance, namely the GFS and NAM MOS, verified
best in this regime yesterday when most model output had a cool
bias. We expect surface warming to be maximized once again today
so we chose to use the 00 UTC GFS and NAM-based MOS to construct
forecast highs. We then strongly weighed forecast humidity values
to recent HRRR output as we are often able to exploit its normal
dry dewpoint bias in cases where steep low-level lapse rates and
uninhibited insolation exist in the spring season. Thus, forecast
humidities this afternoon are in the teens over most of western
and central ND. Light winds will mitigate fire weather concerns
despite the low humidity.

Finally, this scenario will likely once again promote efficient
radiational cooling tonight, with forecast lows once more in the
20s F over most of western and central ND.

.LONG TERM...(Saturday through Thursday)
Issued at 334 AM CDT Fri Apr 28 2017

Warmer, but seasonably cool weather is still forecast for Saturday
through Thursday. Forecast highs are mainly in the upper 50s and
lower 60s F. A chance of precipitation also remains in the cards
for the early to middle part of next week.

The 00 UTC deterministic and ensemble global model guidance stayed
in great agreement with its simulation of larger-scale features,
so our confidence in the long term forecast is actually relatively
high compared to normal. A split 500 mb flow will initially exist
this weekend with a powerful low in the southern branch of the jet
forecast to eject northeastward toward Kansas City by Sunday night,
and into the western Great Lakes by Monday night. A comparatively-
weaker shortwave trough in the northern branch of the jet stream is
expected to spread a chance of showers into western ND beginning
Sunday night, and into the central by Monday. Luckily, it looks
like thermal fields mainly favor rain with this wave. The same is
true with the low precipitation opportunities that will exist from
Tuesday into Wednesday as cyclonic, northwesterly flow becomes
established aloft in the wake of the strong mid-latitude cyclone
that by that time will be moving into southeast Canada. It is
worth noting that neither the 00 UTC ECMWF or GFS extended the QPF
from that dynamically-charged storm far enough northwest for
impacts in central ND or even the southern James River valley, but
we need to mindful that a very low, but non-zero probability may
exist for it to exert influence further west depending on any
potential interaction with the approaching northern-stream wave.
Conversely, extensive deep convection in the warm sector of the
strong storm could also effectively cause an even more easterly
track than currently modeled.


.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 651 AM CDT Fri Apr 28 2017

VFR cigs/vsbys and light winds next 24hr at all terminals.




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