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Office: TFX

FXUS65 KTFX 211612
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1012 AM MDT Mon May 21 2018

.UPDATE...

Large upper low still looks to bring widespread showers and
thunderstorms today. Morning clouds may delay the best instability
until later this afternoon...so made some slight adjustments to
pops given that and current radar trends. Moderate instability is
still expected...especially from Great Falls to Lewistown and
north to Havre A lack of shear will limit the strength of storms
today...although moist dewpoints in the low 50s will contribute to
sustainability. These higher dewpoints may even bring a humid feel
before storms develop. Main threats today will be slow moving
storms...bringing potentially brief heavy rain to some areas. Some
gusty winds and perhaps small hail is also possible in the most
unstable areas mentioned. Temperatures will remain above normal
and in the 70s for today. Anglin

&&

.SYNOPSIS...

A low pressure system currently moving into the Great Basin will
send a series of weak weather disturbances as well as increasing
moisture north into the region through the next several days.
Showers and thunderstorms are likely to develop today and again
each afternoon and evening through Wednesday with greater coverage
of thunderstorms over southwest and central Montana. Some storms
could produce locally heavy precipitation.

&&

.AVIATION...
Updated 1128Z.

VFR conditions are generally expected for the forecast area through
at least 12Z Tuesday. However, brief periods of MVFR ceilings and
mountain obscuration are possible with heavier showers in some
thunderstorms between 18Z and 06Z. A moist southwest flow aloft will
spread mid and high level cloudiness with a few mountain showers
over the forecast area through 18Z. A disturbance in this flow,
paired with increasing instability, will then spread showers and
scattered thunderstorms northeast over the area from 18Z through
00Z. Some storms could produce wind gusts of up to 35 kt. Storms
will likely decrease from the west after 02Z as the airmass becomes
less unstable, but scattered showers will likely linger overnight
with more widespread lower VFR ceilings.  Coulston

&&

.HYDROLOGY...

Flood Warning for Big Hole River (NW Madison/NW Beaverhead Counties)
through 3 pm MDT Monday (today): The river gauge reading near
Melrose has fallen somewhat Sunday night, but it remains just above
flood stage. The current forecast has it staying above flood stage
on Monday and rising to Moderate flood stage Thursday evening, where
it could remain into next week. Will therefore change the warning
wording to be Until Further Notice.

Flood Warning for the Lincoln area (west central Lewis and Clark
County): No significant changes to the ongoing flood situation there
have been reported recently. Again, this warning is set to expire
this afternoon, but it may need to be extended if, after
coordinating with local officials, the flooding remains significant.

Flood Watch for the Jefferson River at Three Forks (along the
borders of Jefferson, Madison, Gallatin, and Broadwater Counties):
The river level has remained steady just below flood stage, but it
is forecast to exceed flood stage Monday evening, likely due to
snowmelt and shower/thunderstorm runoff, possibly reaching Major
flood stage on Thursday.

Additional locations may approach or exceed flood stage Tuesday or
Wednesday and may require further flood highlights if Monday
morning's forecasts are consistent with those from Sunday night. We
are monitoring the Missouri headwaters area (including the Missouri
River near Toston and the Gallatin River near Logan) and the Sun
River (near Simms). Coulston

&&

.PREV DISCUSSION...
/ISSUED 528 AM MDT Mon May 21 2018/

An upper level trough extends south though the western US this
morning with closed low forming over southern CA and moist
southerly flow with embedded disturbances lifting north through
the eastern Great Basin into MT. Upper low tracks slowly NE into
NV Tuesday, then opens/weakens while lifting NE across MT
Wed/Thurs. An upper level ridge amplifies over the interior
western US Fri and Sat before another trough moves in from the
W/SW late next weekend.

A weak weather disturbance lifts N/NE across the region today in
moistening southerly flow aloft. While a few showers are possible
this morning, thunderstorm development should commence this
afternoon over the higher terrain of central and SW MT with
coverage increasing late this afternoon and evening as the upper
level wave moves across the region. Cloud cover could be a
somewhat limiting factor to convective potential today over
western areas. With wind shear and steering flow fairly weak (10-
20 kts)and available moisture (PWATs) relatively high, main
concern with convection this afternoon will be locally heavy
precipitation, especially this evening as flow aloft further
weakens. Similar concerns are anticipated with convection Tuesday
and Tuesday night with greatest concentration of
shower/thunderstorm activity focused over the southern half of the
forecast area. Wednesday holds the best potential for widespread
showers and thunderstorms across most of the forecast area as the
remnant mid-level circulation from the SW US drifts NE across MT
with relatively deep easterly flow aloft, though main forcing
looks to track just east of central MT. System exits to the NE
Thursday with drier conditions developing Friday into early this
weekend. Temperatures remain slightly above seasonal averages
through much of this week with stronger warming late this week
into the weekend, potentially bringing some the warmest
temperatures of the season so far by Saturday. Looking ahead to
the later portion of the upcoming holiday weekend, models are
coming into somewhat better agreement on a fairly vigorous trough
to shift east across the region during the Sun/Mon timeframe.
Hoenisch

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  75  52  73  51 /  30  60  30  20
CTB  73  47  76  48 /  20  20  20  10
HLN  71  49  73  50 /  40  50  60  50
BZN  71  48  72  48 /  30  30  60  60
WEY  63  33  66  37 /  20  20  50  60
DLN  68  43  69  46 /  40  40  40  60
HVR  79  52  78  52 /  20  30  20  10
LWT  72  48  68  48 /  50  50  60  40

&&

.TFX WATCHES/WARNINGS/ADVISORIES...

Flood Warning in effect until 345 PM MDT Monday for the Lincoln area
of west-central Lewis and Clark County.

Flood Warning in effect until 300 PM MDT Sunday for the Big Hole
River in Beaverhead and Madison counties.

Flood Watch until further notice for the Jefferson River in
Broadwater...Gallatin...Jefferson and Madison Counties.

&&

$$

http://www.weather.gov/greatfalls



Office: MSO FXUS65 KMSO 211040 AFDMSO Area Forecast Discussion National Weather Service Missoula MT 440 AM MDT Mon May 21 2018 .DISCUSSION... A daily cycle of afternoon and evening convection will exist across the Northern Rockies through Wednesday. Expect showers and thunderstorms by each mid afternoon, with localized brief, heavy rainfall, cloud-to-ground lightning, and some erratic winds near the strongest storms that develop. Thunderstorm potential will fade by sunset, as the atmosphere stabilizes. Temperatures will be seasonally mild through midweek as well. Temperatures will begin to warm for the later half of the week, as a ridge of high pressure begins to build over the region. By Friday, highs in the upper 70s to low 80s will be possible for many areas across central Idaho and western Montana. The combination of warm afternoon temperatures and weak disturbances moving through the ridge will allow the development of more afternoon thunderstorms. Overall this whole week is looking fairly warm and convective, thus if you are planning to be outdoors during the afternoons, keep an eye on the sky. && .AVIATION...Light winds and variable cloudiness will be present across the Northern Rockies through 21/1800Z. Random, brief, and mostly light rain showers will be drifting across the air space during this time. 21/1800Z through 22/0300Z, more significant showers and thunderstorms will develop. Thunderstorms are possible at all area terminals, accompanied by heavy rain, lightning, and erratic winds. Ceilings will lower with passing thunderstorms. Terrain obscurations will vary from isolated (through 21/1800Z) to frequent (near 22/0300Z). Terrain obscurations will trend downward after 22/0300Z. && .MSO WATCHES/WARNINGS/ADVISORIES... MT...None. ID...None. && $$
Office: BYZ FXUS65 KBYZ 211503 AFDBYZ Area Forecast Discussion National Weather Service Billings MT 903 AM MDT Mon May 21 2018 .UPDATE... No update for inherited forecast this morning. Expect high temperatures to approach 80 degrees in most locations. Expect to see showers and thunderstorms developing in high terrain this afternoon and spread into central plains by this evening. Showers are expected to spread eastward overnight. AAG && .SHORT TERM...valid for Tdy and Tue... Active weather coming the next few days. We will likely see areas of heavy rain, so stay alert to river/stream levels. Water vapor imagery shows trof developing into a low over the great basin, and weaker northern split of this trof moving from BC into Alberta. There is currently a weak shortwave clipping our far west which brought a few showers to the Beartooth/Absarokas over the past few hours, but this activity is dissipating. The remainder of the morning should be mostly dry. Expect an uptick in convective activity later today as we begin to introduce some synoptic scale ascent from the west. Scattered showers/t-storms this afternoon and evening will impact our west and central parts. Shear is quite weak and these storms will be non-severe, but some brief heavy rain, small hail and gusty winds are possible. Latest high res models suggest best potential for strong-ish storms w/ a decent cold pool will develop over the Belts/Snowies and spread across our northwest cwa including Ryegate and Roundup. Some weaker activity may persist overnight across our north as a disturbance tracks through north central MT. Things get more interesting Tuesday afternoon and night as southwest low lifts toward us and ascent/moisture increase. Tap of gulf moisture will push precipitable waters to anomalously high levels, up to an inch or more across our eastern cwa. A stronger shortwave will lift north thru WY Tuesday night. All of this will result in periods of showers and thunderstorms through Tuesday night (i.e. not just diurnally), with locally heavy rain a good possibility. Will need to watch for potential impacts on already high rivers and streams. Also, although it is modest, models have trended a bit higher with bulk wind shear across our east. A few severe thunderstorms (wind/hail) are possible Tuesday afternoon and evening, mainly east of Rosebud County. Anyone with outdoor plans Tuesday evening should be ready for weather impacts. One fly in the ointment could be convection-induced cooling from storms to our north tonight, which could impact the lower air mass especially across our west on Tuesday. This boundary could also be something to watch for convective initiation on Tuesday. Temperatures will be quite mild today with 70s to lower 80s for highs. Tuesday should be a bit cooler but still above normal. JKL .LONG TERM...valid for Wed...Thu...Fri...Sat...Sun... Latest model runs continue to show an active pattern in the extended. A trough over the Great Basin Tuesday will move north into the Northern Rockies. This will produce showers and thunderstorms Wednesday. Shear continues to be relatively low, so not expecting organized severe thunderstorms. However, there could be some stronger storms towards the Dakotas. The bigger concern is preciptable water is still around an inch and without much steering flow these storms they could produce heavy rainfall. This heavy rainfall could impact the river levels as we are nearing the peak of snowmelt runoff. Precip should start to decrease Thursday as the trough moves north and east. Friday could see some lingering energy produce some afternoon showers and thunderstorms, mainly over the mountains. Saturday should be dry, with only a slight chance of a shower over over the mountains. Sunday will start to see more energy work in from the next system moving off the Pacific. The models are not in as good agreement on the track of this energy so have some PoPs, but not much more than climatology. Wednesday will likely be the coolest day in the extended with the showers and increased cloud cover, but highs will still be near normal. Through the remainder of the extended will see temperatures warming with highs by the weekend in the mid to upper 80s. Hitting 90 degrees at a few locations is not out of the question on Sunday. Reimer && .AVIATION... In general expect VFR conditions to prevail. This afternoon scattered showers and isolated thunderstorms will develop from KBIL and KSHR west, with the best chances around KLVM and K6S0. These storms could produce some MVFR/IFR conditions and mountain obscurations. Reimer && .PRELIMINARY POINT TEMP/POPS... Tdy Tue Wed Thu Fri Sat Sun ------------------------------------------------------- BIL 079 054/075 054/070 054/078 056/082 056/083 056/084 2/T 24/T 77/T 42/T 12/T 11/U 11/U LVM 075 048/071 049/065 049/073 050/077 050/080 050/078 4/T 47/T 78/T 54/T 22/T 21/U 12/T HDN 081 052/079 054/072 054/079 056/083 056/084 058/085 1/U 23/T 77/T 43/T 11/B 11/U 11/U MLS 080 055/079 058/075 057/077 058/082 058/083 058/084 0/U 22/T 56/T 43/T 11/U 10/U 11/U 4BQ 080 054/081 057/074 055/077 057/080 056/083 057/084 0/U 24/T 66/T 32/T 11/U 11/U 11/U BHK 076 051/078 056/074 056/076 056/079 056/081 055/080 0/U 14/T 66/T 44/T 12/T 00/U 11/U SHR 078 050/075 052/071 051/075 052/078 052/080 053/082 2/T 34/T 87/T 42/T 22/T 11/U 11/U && .BYZ WATCHES/WARNINGS/ADVISORIES... MT...None. WY...None. && $$ weather.gov/billings
Office: GGW FXUS65 KGGW 211458 AFDGGW Area Forecast Discussion National Weather Service Glasgow MT 858 AM MDT Mon May 21 2018 .DISCUSSION... Morning Update: No changes made to the forecast for the morning update. PREVIOUS DISCUSSION: Expect a relatively warm and dry weather day across northeast Montana for the most part. An upper level shortwave will approach this afternoon from the west. Dynamic forcing will enhance ascent, especially across upslope terrain areas upstream of the CWA. That said, expect any convection to spill into the western zones by this evening. Nam Bufkit forecast soundings nicely show a little instability, and CAPE as high as 500 J/kg near Jordan this afternoon and evening. At this time, little in the way of 0-6 km shear should prevent stronger storms, but brief heavy downpours, as well as breezy conditions may accompany some of the multicellular structures that evolve across the area tonight. While afternoon and evening convective activity will again be possible on Tuesday, right now the focus is on Wednesday and Thursday as being the most active. An upper shortwave trough and associated vorticity maxima over northern Wyoming will lift into southern and central Montana on Wednesday and eventually eastern Montana by Thursday. Meanwhile, expect surface low pressure to track well southeast of the CWA into the Plains during this time. Low and mid level moisture will increase ahead of this shortwave, and forecast Bufkit soundings show a more pronounced nice veering wind profile with increasing levels of 0-6 km sheer across locations within the CWA. Nice inverted V sounding in the low levels in the southeast zones in particular in the presence of low level shear suggest wind potential with any convection. While models show in excess of 1000 J/kg surface-based CAPE over these locations on Thursday, any elevated convection over complex terrain and across portions of the CWA may be sustained well into the evening hours. In short, will be closely monitoring Wednesday and Thursday for at least an isolated stronger or possibly severe storm over portions of NE Montana, and if anything, with precipitable water values between 1.00" and 1.50" do think locally heavy rainfall may result as well. The forecast beyond the midweek was trended toward consensus model blends given the usual spread in guidance at larger time scales and lower predictability of mesoscale features. Early indications, however, suggest a building ridge over the western U.S. with warmer and dry weather conditions. Maliawco && .AVIATION... Flight Category: VFR. Synopsis: Sunny skies and dry weather are expected this morning. Late this afternoon and tonight, expect increasing cloudiness as an upper level disturbance approaches the region, though most showers and storms will remain upstream of the area terminals through the period. Wind: Expect winds today out of the southeast and under 10 kts, turning to the northwest tomorrow morning and remain under 10 kts. Maliawco/Bigelbach && .GLASGOW WATCHES/WARNINGS/ADVISORIES...NONE. && $$ weather.gov/glasgow