Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 200947

Area Forecast Discussion
National Weather Service Great Falls MT
347 AM MDT Tue Mar 20 2018


Dry conditions are expected for most of North Central and
Southwest Montana today, with the only exceptions being along the
Continental Divide and over the mountains of Southwest Montana
where light showers will be possible. High temperatures will warm
each day through the day on Wednesday, with some locations nearing
60 degrees by the middle of the work week. An upper level
disturbance then moves into the Northern Rockies to end the work
week, which will bring a return of precipitation to most of North
Central and Southwest Montana.



Today through Thursday...main forecast concerns over the short term
period is temperatures and precipitation chances over the higher
terrain of Southwest Montana.

Short term period begins with light northwest flow at H500 across
the Northern Rockies, with a subtle and weak wave rippling through
the flow and over Southwest and North Central Montana during the
afternoon hours today. Further south and to the west, H500 ridge
over California/Great Basin/Desert Southwest this morning, will
begin to build east and north throughout the rest of Tuesday and
into the day on Wednesday. Available moisture for the day today will
be very limited, with PWATs generally ranging around 0.15" to 0.25"
(which is around 1-2 standard deviations below normal values per
NAEFS analysis). This limited available moisture should help to keep
most of the CWA dry during the day today despite the passage of the
aforementioned weak wave, however, H700 frontogenetical forcing
across Southwest Montana combined with orographic lift may be enough
to squeeze out some precipitation this afternoon/evening. Any
precipitation across Southwest Montana and along the Continental
Divide (where orographic lift will also help to squeeze out the
limited moisture) over this time frame will be showery in nature,
and likely produce only minor QPF amounts. In addition, weak lee-
side surface trough will sharpen slightly during the morning hours
today, which will increase the surface pressure gradient along the
Rocky Mountain Front and mountains of Central Montana, aiding in
breezy southwest winds for areas along and west of the Interstate 15
corridor and generally along/south of a Great Falls to Lewistown
line. High temperatures south and west of these aforementioned areas
are expected to warm into the 40s and low-50s (upper 20s to 30s in
the mountains), with the 30s expected north and east of this line.

By Wednesday morning, H500 ridge will be firmly in place across the
Northern Rockies/Northern High Plains, with 1000-500mb thickness
values rising to the order of 540-550dam across North Central and
Southwest Montana respectively. Strong mid-level warm air advection
will occur throughout the day on Wednesday, ahead of an digging and
deepening H500 trough/closed low moving from the Gulf of Alaska and
into the Pacific Northwest. This combined with light southwesterly
winds across most of North Central and Southwest Montana, should
push daytime high temperatures into the mid-40s to upper 50s for the
day on Wednesday, generally along and southwest of a Lewistown, to
Fort Benton, to Cut Bank line. Areas northeast of this line can
expected highs in the 30s, with the coolest readings over Hill and
Blaine counties where a substantial snow pack still resides. As the
ridge axis begins to shift further east during the afternoon/evening
hours on Wednesday, H700-H500 flow will become increasingly
southwesterly in nature. This southwesterly flow will being to usher
in Pacific moisture across Southwest Montana and along the
Continental Divide. Orographic lift combined with that increasing
moisture, will help support increasing snow/rain showers across the
aforementioned region. H500 ridge amplifies further across the High
Plains of the United States/Canada Wednesday night and during the
day on Thursday, as the H500 trough begins to move onshore and
towards this Intermountain West. H700 temperatures on Thursday warm
to 1-2 standard deviations above normal per NAEFS analysis over
North Central and Southwest Montana, which could lead to even warmer
temperatures than the day prior. However, increasing clouds and
precipitation chances have me concerned that guidance may be to
aggressive with temperatures, so have decided to keep the values
similar to that of Wednesday (plus or minus a couple of degrees). As
just stated, precipitation chances will be on the increase during
the day on Thursday, especially across all of Southwest Montana and
along the Continental Divide. Enough instability may even be present
on Thursday for a few rumbles of thunder. - Moldan

Thursday night through Monday...The main concern during this
timeframe continues to revolve around what impact a developing
atmospheric river event will have on the local area.

Synoptically, the late-week period will feature a developing SW flow
aloft as a large and anomalously strong upper low over the Pacific
begins to move inland. Meanwhile, an anomalous ridge will be
centered over Mexico. In between these two features, a strong upper
level jet will allow a surge of moisture to crash into the West
Coast and eventually move NE across parts of the Rockies. It is
worth noting that the clustering of ensemble guidance has shifted a
bit further north with this stream of moisture, which may put a bit
more emphasis for higher precip amounts over far SW MT (south of I-
90). Ensemble guidance suggests precipitable water values within
this plume of moisture may reach the 99th percentile for this time
of year.

So, what does all this mean locally? For parts of SW MT, especially
along the MT/ID border, this may translate into a decent
precipitation event. The higher moisture content may also be
accompanied by milder temperatures, allowing a mix of rain and snow
(as opposed to all snow). With it being several days out still,
there is still some potential for the axis of best moisture to
shift, but given the recent lower elevation runoff, it bears
watching as it could lead to an increased risk of flooding for some
valley locations south of I-90. Further north, the models have
actually backed off on precipitation amounts, possibly due to more
pronounced downsloping effects. In the wake of the THU/FRI system, a
gradual cooling trend is expected along with generally unsettled
conditions as the upper low moves overhead. In fact, this weekend
very much resembles the unsettled pattern we dealt with last week
with periods of rain and snow. MARTIN


Updated 0520Z.

VFR conditions are expected at most terminals through the duration
of the 2006/2106 TAF period, with the only exception being the KHVR
terminal. At the KHVR terminal, dense fog reducing visibilities to
LIFR conditions is expected to persist through the remainder of the
overnight hours. A gradual improvement in visibilities and ceilings
is expected then during the mid-morning hours at the KHVR terminal,
with VFR conditions resuming around 16z. Otherwise, breezy
southwesterly winds will be experienced at the KGTF and KCTB
terminals tonight and during the day on Tuesday. - Moldan



Moderating temperatures will lead to increased runoff once again
this week, especially through Friday. Initially, precipitation
amounts look to be light through mid-week, but attention will then
turn towards the potential of a decent precipitation event,
especially far SW MT. While it is unlikely that all of the
precipitation will fall as rain, at least some will which combined
with less cool temperatures could locally enhance runoff and hydro
concerns. For now, we will cover this potential with a Hydrologic
Outlook, but will continue to evaluate the situation and model
guidance to see if any flood watches are needed. MARTIN


GTF  47  27  51  29 /   0   0   0  10
CTB  40  21  44  22 /   0   0   0  10
HLN  51  27  56  31 /   0   0  10  10
BZN  45  23  49  32 /  10   0  10  20
WEY  34  18  38  27 /  20  30  50  40
DLN  47  22  50  30 /  10  10  10  20
HVR  34  15  35  20 /   0   0   0  10
LWT  43  23  50  29 /   0   0   0  10





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