Area Forecast Discussion
Issued by NWS Billings, MT

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FXUS65 KBYZ 201603

Area Forecast Discussion...Updated
National Weather Service Billings MT
1003 AM MDT TUE SEP 20 2016


Energy embedded in southwest flow aloft has allowed for an area of
showers to persist across north central zones this morning. This
area of showers will gradually move off to the northeast through
the morning. Another disturbance is still on track to move across
the region bringing showers and isolated thunderstorms to the
area starting this afternoon and continuing into the overnight.
Best chances still look to be along and north of a Livingston-
Billings-Miles City line. Have adjusted pops and sky cover to
current trends. Otherwise, the current forecast is handling the
situation well and the rest of the forecast remains on track. The
previous discussion is below. STP


.SHORT TERM...valid for Tdy and Wed...

Today looks to be the last of the mild days until possibly next
Sunday or Monday. A large upper low and trough will approach the
Pacific Northwest coast today and move onshore during the day
Wednesday. This will allow an upper level ridge to buckle over
the Northern Rockies and keep the jet stream across southern
Canada. Water vapor imagery is currently showing a moist flow
streaming in off the Pacific across Idaho and Montana. A weak
disturbance within this flow is moving over Southwest Montana
where an area of showers and isolated thunderstorms have
developed. As a result, I did add a slight chance for a light
shower or sprinkle across all but the far southeast areas of our
forecast area for this morning. Satellite is also showing plenty
of mid and high level clouds heading toward the area so look for a
partly to mostly cloudy day for most areas. Another disturbance
approaches the area late this afternoon and through the overnight
hours bringing better chances for showers and isolated
thunderstorms with the best chances along and north of a
Livingston-Billings-Miles City line.

The upper low and trough move onshore Wednesday bringing a
southwest flow aloft across the forecast area. A stronger
disturbance within the the flow is progged to move across
southwest and north central Montana during the afternoon and
evening, and as a result, we will continue with a chance of
showers and isolated thunderstorms along and north of a
Livingston- Billings line. A strong area of Q-Vector convergence
moves across the north central and northeast portions of the state
Wednesday evening and overnight and this is where models put the
highest qpf. However, it still looks as if our western and
northern areas from Park to Musselshell Counties will get in on
this so I have increased pops into the likely category for this
time period.

Highs today will be mainly in the 70s for one more day before
cooler air arrives Wednesday when temperatures will be below
normal with upper 50s to middle 60s common. Hooley

.LONG TERM...valid for Thu...Fri...Sat...Sun...Mon...

Quick overview of the storm evolution the end of this week starts
with Thursday the upper low over western Great Basin with flow
over southern Montana and northern Wyoming backing and becoming
diffluent especially Thursday night. Surface pressures fall over
Wyoming tightening upslope gradients.

Friday Upper low moves into western Wyoming and southwest
Montana. Flow aloft becomes less diffluent with surface low
pressure shifting into western Nebraska and South Dakota causing
less upslope but increasing wrap around flow into southeast

Moisture becomes unseasonably abundant on Thursday and persists
into Friday so any lift or minor convective mechanism will be
efficient at producing rainfall. By Friday night numerous reports
of 1 to 2 inches of rainfall are expected. SREF Plume means from
the last two runs are consistent in projecting .25 for Billings
Thursday and about .65 Thursday night. Surprisingly CIPS analogs
seems to be less aggressive in QPF amounts but suspect they are
not as strong with the Precitable Waters. Snowfall amounts will
be less impressive as the system is pretty warm and 8000 ft
probably would be the floor of significant snowfall.

Models in good agreement through Friday and then diverge for
Saturday. ECMWF and GFS have the low tracking into eastern Montana
for Saturday while the Canadian and UKMET send the low digging
towards the desert SW. GFS has an inclination of the southward
move but a bit later. Conclusion is that models are trying to
generate a split at some point so how wet and cool Saturday will
be is a low confidence forecast. Sunday and Monday do look dry.



Scattered showers can be expected today as a warm front moves into
the area. Conditions should stay VFR but a few higher mountain
tops could become obscured. The best chances for shower activity
will be north and west of KBIL. Winds will be light today.



    Tdy Wed     Thu     Fri     Sat     Sun     Mon
BIL 072 047/059 047/055 048/054 045/057 043/067 048/071
    2/W 23/R    66/R    86/R    55/R    21/B    01/U
LVM 070 045/058 043/055 044/051 041/057 040/066 044/070
    3/T 33/R    77/R    76/R    54/R    21/B    01/U
HDN 077 049/065 049/062 049/058 044/058 043/069 046/071
    2/W 12/R    55/R    86/R    65/R    31/B    01/U
MLS 078 052/066 050/061 050/061 048/057 043/067 047/072
    2/W 32/R    66/R    87/R    65/R    41/B    01/U
4BQ 080 052/069 051/066 052/064 047/058 042/067 046/072
    0/B 11/B    25/R    65/R    64/R    31/B    01/B
BHK 076 050/065 049/058 047/060 047/057 043/063 044/069
    1/B 31/E    34/R    66/R    65/R    32/W    01/U
SHR 080 045/069 049/068 050/061 043/058 041/067 045/072
    0/B 11/B    35/R    75/R    64/R    31/B    11/U




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