NWS Great Falls, MT wrote its last area forecast discussion before the first event at Jun 14, 10:40 AM MST, 1 hour 20 minutes earlier.
Area forecast discussion, Jun 14, 10:40 AM MST, 1 hour 20 minutes before
Area Forecast Discussion
National Weather Service Great Falls MT
1140 AM MDT Mon Jun 14 2021
Aviation Section Updated.
.SYNOPSIS...
Hot and dry conditions will prevail across the Northern Rockies
through Tuesday, before a strong Pacific cold front brings falling
temperatures and windy conditions for the day on Wednesday. Record
high temperatures will be in jeopardy of falling at numerous
locations both today and on Tuesday as highs rise into the 90s to
low 100s.
&&
.UPDATE...
The morning update has been published. The primary change was to
issue a Red Flag Warning over our southeastern fire zones for
Tuesday. This was done in coordination with the neighboring
offices and some of our fire weather partners. The southeastern
portions of the warned area have a high probability of observing
red flag conditions, while areas to the northwest undergo
efficient drying/curing of vegetation. In addition to that, a
westerly wind shift is expected over the plains on Tuesday
afternoon/eve, which could create containment difficulties for any
new fire starts.
The remainder of the forecast has remained unchanged. Still
looking for hot conditions today with a chance for isolated storms
this evening, mostly along the Divide and over higher terrain of
Central/North-central MT. -RCG
&&
.AVIATION...
1140 AM MDT Monday Jun 14 2021 (14/18Z TAF period)
VFR conditions prevail under mostly clear skies. Expect some
scattered cumulus cloud development over the central MT mountains this
afternoon with a small chance for an isolated thunderstorm or two to
move north out of those areas this evening. Otherwise little cloud-
cover is expected through tonight. Surface winds will increase today
from the east to southeast across north-central MT and from the south
across southwest MT.
&&
.FIRE WEATHER...
Near record heat and low humidity will accelerate the drying of
fuels early this week. Gusty southwest winds are expected Tuesday
afternoon, particularly across southwest MT where afternoon
humidity will fall to around 10-15%. Further north across north-
central MT, humidity will not be as low today, but is expected to
fall to 10-20% Tuesday afternoon as winds increase from the
south and a wind shift to the west is likely late Tuesday or
Tuesday night in association with a mainly dry cold front. The
increase in winds in addition to the heat and dryness will raise
fire weather concerns across the area and a Red Flag Warning has
been issued for Tuesday for the south and eastern fire weather
zones. The degree of drying and curing of fuels is less certain
further north, but still need to be monitored for fire weather.
&&
.PREV DISCUSSION...
/ISSUED 337 AM MDT Mon Jun 14 2021/
Today through Tuesday...primary concern and biggest story of the
period will be the heat.
H500 ridge, with an axis extending north from Eastern Montana to
over the Northwest Territory of Canada, is firmly in place over
much of the western halves of Canada and the CONUS this morning.
With 1000- 500mb thickness values expected to climb another
50-100m today, expect temperatures to climb another 7 to 10
degrees from that of yesterday's highs, which will place most
locations in the mid-90s to low 100s for highs today. Have
maintained the Heat Advisory for Hill, Blaine, Chouteau, and
Fergus counties as this is where high temperatures are expected to
peak in the low 100s at numerous locations both today and on
Tuesday. While other locations outside of this Heat Advisory are
expected to reach into the low 100s today, they are not expected
to do so on Tuesday. Therefore give the brevity of the heat (at or
over 100 degrees) at these locations outside of the current Heat
Advisory I have decided to withhold from expanding the on-going
advisory further west or south. Another subtle mid- and upper
level wave (denoted by weak height falls of ~10m to 20m) is then
expected to lift northeast within the upstream side of the ridge
late this afternoon and evening, which combined with steep lapse
rates and sufficient moisture could spark off another round of
late afternoon and evening showers and thunderstorms. Best chance,
albeit a low chance (generally 15% to 30%), for these showers and
thunderstorms will be along and north of a White Sulphur Springs
to Helena line (especially initially over the mountains). Any
showers and thunderstorms that do form will lift northeast,
generally producing less than 0.10" of rain.
On Tuesday the H500 ridge axis will shift further east as a longwave
trough begins to dig in over the Pacific Northwest and Northern
California. This will help to shift the hottest temperatures (100 to
near 110 degrees) further east to over our far eastern zones (Hill,
Blaine, Chouteau, and Fergus Counties), Eastern Montana and the
Dakotas; however, widespread highs in the low to mid-90s will still
be common across the remainder of Southwest through North Central
Montana. Increasing low to mid-level south to southwesterly flow on
Tuesday will lead to increasing surface winds across the entire
region with diurnal mixing, especially in the north-south orientated
valleys of Southwest Montana where terrain funneling will enhance
speeds. I am concerned that deep afternoon mixing through H700 to
near H500 over Southwest Montana, where wind speeds of 35 to 55kts
will be present, combined with the orientation of the wind to the
terrain (terrain funneling) could produce a 6-9hr period (between
noon and 9pm) of High Wind criteria speeds and gusts across
Beaverhead and Madison counties. Have withheld issuing a High Wind
Watch with this forecast package, but will relay my concerns to
future shifts. A Pacific cold front will then begin to move across
Western Montana late Tuesday afternoon as a surface low develops
over Northern Montana/Southern Alberta and quickly ejects northeast
to Central Saskatchewan (Tuesday evening). - Moldan
Tuesday night through next Monday...primary concern and focus will
be on the potential for high winds on Wednesday.
The Pacific cold front will quickly slide east and across the CWA
Tuesday night, shifting winds to the west in addition to increasing
speeds. While the strongest H850-700 winds will be present over
Central and North Central Montana Tuesday night, questions about how
deeply mixed the region remains during the overnight hours creates
some doubts that high winds will materialize despite wind speeds of
35-55kts (H850-700). If these winds were able to mix down, believe
that the Rocky Mountain Front, Eastern Glacier, and western portions
of Toole County would be most at risk of seeing high winds given a
moderate pressure gradient orientated along the Continental Divide
and the potential for mountain wave activity given weak ridgetop
stability. By Wednesday the H850-700 flow will begin to weaken to
around 30-40kts, all the while shifting north to over the Hi-Line.
While lower in overall speeds, deep mixing through these levels
(along and north of the Hi-Line) Wednesday afternoon does raise
concerns that some locations could reach High Wind criteria values
on sustained wind speeds, especially given deep unidirectional
flow through H700 combined with subsidence on the backside of the
departing upper level shortwave. Have withheld on issuing any High
Wind highlights for this timeframe, but as is the case for
Southwest Montana on Tuesday I will pass these concerns along to
future shifts. While a few isolated to scattered showers and
thunderstorms are possible along the Continental Divide Tuesday
night with the passage of the Pacific cold front, the front as a
whole will be fairly dry, only bringing a drop in temperatures for
the day on Wednesday and through the remainder of the work week.
- Moldan
&&
.PRELIMINARY POINT TEMPS/POPS...
GTF 99 60 92 49 / 10 10 0 10
CTB 97 56 83 48 / 20 30 10 10
HLN 102 60 95 53 / 10 0 0 0
BZN 100 53 97 48 / 0 0 0 0
WYS 90 42 86 40 / 0 0 0 0
DLN 96 54 91 45 / 0 0 0 0
HVR 100 64 100 53 / 10 10 0 10
LWT 96 59 97 51 / 10 20 0 0
&&
.TFX WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from 10 AM to 10 PM MDT Tuesday Gallatin
National Forest.
Red Flag Warning from 9 AM Tuesday to midnight MDT Tuesday night
Central and Eastern Lewis and Clark National Forest Areas...
Chouteau and Fergus Counties...Helena and Townsend Ranger
Districts of the Helena National Forest...Hill and Blaine
Counties.
Heat Advisory until 8 PM MDT Tuesday Blaine...Chouteau...
Fergus...Hill.
&&
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