Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 170911
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
311 AM MDT Thu May 17 2018

.SHORT TERM...(Today and tonight)
Issued at 233 AM MDT Thu May 17 2018

Upper level trough over the Great Basin will produce a southwest
flow aloft over the Central Rockies through tonight. At the
surface, a low pressure trough will form over eastern Colorado and
act as a dry line. Southeast low level winds to the east of it
will advect moisture into eastern Colorado with dew points in the
50s. This combined with surface temperatures in the 80s will
result in surface based CAPE up to 3000 J/kg over eastern
Colorado. Along with this instability, there will be enough shear
to produce super cell thunderstorms. Main threat will be large
hail, golf sized or slightly larger. Can`t rule out a tornado,
especially if a super cell can interact with boundary.

Right now models show the dry line and the chance for severe
thunderstorms east of a line from the Wyoming/Nebraska border
south to Fort Morgan and to Limon. Models are in good agreement
showing storms developing over the northeast corner of the state
early this evening. They also indicate storms may reform in this
area overnight due to the convergence along the dry line/surface
trough. If this happens a couple areas could see 2-3 inches of
rain and minor flooding issues. From Akron south along the dry
line, some models show convection while others don`t. If storms
are able form, they may become severe. Will have lower pops in
this area where confidence in storm development is lower.

West of the dry line, including the Front Range, there will be
little to no chance for thunderstorms today due to the dry airmass
at the lower and mid levels. It will be slightly warmer today
with highs in the lower to mid 80s over northeast Colorado.

.LONG TERM...(Friday through Wednesday)
Issued at 233 AM MDT Thu May 17 2018

Friday and Friday night, the flow aloft will be southwesterly
ahead of an open wave mid level trough over northern UT early
Friday. This system will pass over northeast CO Friday night. In
the mid levels, weak to moderate qg ascent will be over the
region through that time. At the sfc, a low over southeastern CO
early Friday will strengthen through the day, with increasing
northeast to northerly surface winds developing over the northeast
plains of CO by 00z Saturday. The sfc low remains stationary
through the day Friday then lifts northeast into central KS by 12z
Saturday. GFS/NAM12 computed CAPES range from 2000-3000 j/kg
Friday afternoon. The forecast GFS/NAM12 soundings show
directional shear with descent speed shear as well as the sfc low
strengthens late Friday afternoon. SPC keeps the I-25 Corridor
and northeast plains in slight risk for severe on Friday and this
all looks reasonable. Large hail and damaging winds the main
threat but a weak tornado is also a possibility. Best window for
the severe thunderstorm threat in the 21z-03z period, along and
south of a cold front that will be dropping out of WY late in the
afternoon. Friday night, better synoptic support in the upper
levels, with rainfall becoming more widespread across the
northeast plains overnight. Embedded thunderstorms as well, but
should be more isolated after 06z Saturday. In the mountains, snow
levels will lower to around 9500 feet by 12z Saturday. Could see
advisory snowfall amounts above 10 thousand feet, with 4-8 inches
possible in a few locations. By Saturday, cooler more stratiform
rainfall expected across the northeast plains in a post frontal
airmass. Will keep a slight chance of thunderstorms over the
higher terrain and in the far northeast corner in proximity to
the upper low, but go with primarily rain showers elsewhere. No
sfc based CAPES generated by the models, and the forecast
soundings looks quite stable but saturated through the day.
Saturday night, precipitation will decrease from west to east with
upper level ridging and subsidence developing into Sunday. It
will be warmer on Sunday with the ridge axis expected to shift
east into KS by 00z Monday. Enough moisture around in the
afternoon with better heating for scattered showers/tstms in the
afternoon south and west of I-25. By late Sunday
afternoon/evening, some of that will shift east of the I-25
corridor and adjacent plains. No big changes yet for early next
week. The warming trend will continue with mid 70s for Denver on
Monday, then around 80 through the middle of the week. Still
unsettled, with a chance of thunderstorms each day. A weak trough
over northern CA/NV will continue to generate weak impulses that
could brush across the state, one on Tuesday night, with another
potential one late Wednesday into Wednesday night.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 233 AM MDT Thu May 17 2018

VFR conditions are expected to prevail through tonight. Southerly
winds this morning are expected to become light by 15Z. Expect a
wind shift to the west-northwest to occur 21-23Z due to deepening
low pressure over eastern Colorado. Eventually a surface low over
southern Colorado intensifies enough to cause winds to switch to
the northeast Friday morning. This will bring a chance for fog and
low clouds after 12Z Friday.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

SHORT TERM...Meier
LONG TERM...Cooper
AVIATION...Meier



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