NWS Grand Junction, CO wrote its last area forecast discussion before the first event at May 20, 6:32 AM MST, 3 hours 28 minutes earlier.
Area forecast discussion, May 20, 6:32 AM MST, 3 hours 28 minutes before
Area Forecast Discussion...CORRECTED
National Weather Service Grand Junction CO
732 AM MDT Sat May 20 2023
...CORRECTED TYPO IN SHORT TERM DISCUSSION...
.SHORT TERM...(Today through Sunday)
Issued at 350 AM MDT Sat May 20 2023
The synoptic pattern featured an amplified mid-level ridge of high
pressure over the West, a deep low over the Great Lakes region, and
a shallow mid-level low pressure trough extending southwestward
from southeast Wyoming to the northern Baja Peninsula. The trough
itself appeared to have to separate circulations, one over north-
central Colorado and the other over northeast Arizona. Meanwhile,
KGJX radar indicated a weakening convective vortex near the Abajo
Mountains in southeast Utah which kept light showers going within
its vicinity through the night, though those returns were
diminishing as of 0930Z.
As mentioned in the previous short term discussion, drier air
infiltrated northeast Utah and northwest Colorado on Friday.
Meanwhile, an anomalously deep moisture surge lingered over the
southern half of the forecast area. The NAEFS ensemble situational
awareness table indicated moisture levels near the 99th
percentile will encompass much of the southern border region of
southeast Utah and southwest Colorado. Precipitable water levels
peak around 0.75" so brief heavy rain is a possibility this
afternoon, though this will largely depend on how much clearing
occurs over the south during the morning. Weak shear and light
steering flow suggests today's thunderstorms will be mainly
short-lived single cells so their threat comes mainly in the form
of lightning, brief heavy rain, modestly gusty outflow winds and
small hail.
Showers and thunderstorms will decrease in the evening as diurnal
cooling allows the atmosphere to stabilize. There's a slight
chance some shower activity will continue overnight, though only
the GFS seemed to favor this idea. Sunday will feature similar
conditions, though drier air will spread a bit farther south
limiting shower/thunderstorm activity along the I-70 corridor with
little discernible change farther south.
Temperatures across the northern half of the forecast area will be
largely unchanged relative to Friday, but will be around 5 degrees
cooler on average across southeast Utah and southwest Colorado due
to increased clouds and showers. Slight warming is indicated for
Sunday, though this is less certain across the south where deeper
moisture will linger.
.LONG TERM...(Sunday night through Friday)
Issued at 350 AM MDT Sat May 20 2023
By Sunday night, a deep trough of low pressure will be located along
the coast of Washington and British Columbia. While energy from this
trough will not reach the West Slope, it will influence the overall
pattern and finally bring a change from the unsettled pattern we've
seen. The ridge that has been over the Great Basin the last several
days will finally start to nudge eastward as the trough deepens
along the West Coast. Monday and Tuesday won't see much change in
sensible weather though, as much above normal moisture remains
pooled across the Four Corners region. The best moisture will be
south of I-70, and therefore the best storm coverage will continue
to be across the southern half of the CWA. By Wednesday, however,
things start to shift. The deepening West Coast trough will be
positioned such that a weak southwesterly jet aloft will push into
the Four Corners region. This southwesterly flow will start to nudge
the moisture pool northward, with PWAT anomalies of 160-200% of
normal overspreading the entirety of eastern Utah and western
Colorado by Wednesday afternoon. This will likely be the day with
the highest shower and storm coverage of the week. Through this
early week period, the main threats with any storms will continue to
be gusty outflow winds, lightning, and brief heavy rain. By
Wednesday night, the strengthening southwest flow will begin to pull
drier air northward into the Four Corners region. Shower and storm
activity on Thursday afternoon will be limited to areas generally
north of I-70, with the southern half of the area seeing warmer,
drier, and windier conditions. By Friday, this drier airmass will
have overspread the entirety of eastern Utah and western Colorado as
southwesterly flow remains in control. These conditions look to
remain through the coming weekend.
Temperatures will be near normal at the start of the long term
period, thanks to continued afternoon cloudiness and storm activity.
Overnight lows will be near normal in areas that see nighttime
clearing, while areas that remain cloudy will see lows run 5-10
degrees above normal. For the second half of the week, with the
drier airmass in place, high temperatures will jump to 5-10 degrees
above normal for late May. Low temperatures will likely remain near
to slightly above normal.
&&
.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 540 AM MDT Sat May 20 2023
Deep moisture across the southern portion of the forecast area
will fuel scattered to numerous showers and thunderstorms today.
As a result, KTEX and KDRO are likely to experience showers and/or
thunderstorms, mainly during the afternoon and evening with
showers in the vicinity of KGUC, KMTJ, KASE and KEGE. Winds will
generally be light with moderate gusts from outflows at the TAF
sites listed previously. Despite showers, VFR conditions are
likely to prevail at all TAF sites. Brief MVFR conditions are
possible at KTEX and KDRO in heavier showers.
&&
.HYDROLOGY...
Issued at 350 AM MDT Sat May 20 2023
Drier northwest flow moved over northeast Utah and northwest
Colorado Friday and will limit shower/thunderstorm activity over
this region again this afternoon. Meanwhile, an anomalously moist
air mass will be found over the remainder of the forecast area.
Convective storms feeding off this moisture combined with
instability aloft will continue to support slow moving
thunderstorms favoring mountainous terrain this afternoon.
Heavy rain remains a threat across the southern half of the
forecast area today and across southwest Colorado on Sunday
according to WPC which has those areas in the marginal risk for
excessive rainfall this weekend. This would support periods of
prolonged rain or bursts of heavy rainfall on existing snowpack.
Forecast hydrographs correlated well with this outlook,
anticipating a sharp rise in rivers along the southern San Juan
foothills over the this weekend.
Main stems (i.e. Yampa, Green, Colorado, Gunnison and Dolores
Rivers) will be influenced by rain on snow driven snowmelt
mentioned previously. However, flashy responses will be absorbed
further downstream. In addition, reservoir spills and controlled
releases will largely dictate levels along major rivers.
Based on this weekend's forecast, flashy spikes are still
possible along tributaries to the Colorado, Gunnison, Dolores and
San Miguel Rivers. Overall, this active weather pattern persists,
and temperatures remain just a touch above normal in the near term.
The short/midterm forecast time line falls well within the norm
for the spring runoff period. Therefore, we expect high elevation
snowpack to continue declining while water levels run near or
above bankfull for major rivers across the West Slope.
&&
.GJT WATCHES/WARNINGS/ADVISORIES...
CO...None.
UT...None.
&&
SHORT TERM...NL
LONG TERM...BGB
AVIATION...NL
HYDROLOGY...ERW/NL