National Weather Service Forecast Discussion

000
FXUS65 KBOU 190251
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
851 PM MDT Thu Oct 18 2018

.UPDATE...
Issued at 831 PM MDT Thu Oct 18 2018

No major adjustments to the grids at this time. Snow showers
moving through Summit County. Roads wet with icy spots near
the Eisenhower Tunnel this evening. Current forecast of 2-5"
looks reasonable, higher end of that range will be more
localized. Some rain showers may spill out of the foothills and
into the I25 corridor. Most of the showers should stay west of
the interstate and dissipate altogether after midnight. Stratus
deck advancing north and west into Lincoln and southern Washington
early this evening, so will increase the cloud cover there. Most
of this will shift eastward after midnight.

&&

.SHORT TERM...(This evening through Friday)
Issued at 109 PM MDT Thu Oct 18 2018

Water vapor imagery shows a trough that is currently rotating
over Utah with a southwest to northeast oriented jet streak
extending from eastern Arizona to Nebraska. This trough will
slowly move eastward through the night and will be picked up
within the northwesterly flow tomorrow. As this system moves
through Colorado tonight, it will create convective showers mainly
over the central mountains south of Grand Lake through northern
Park County. There is growing confidence in this outcome because
there are convective showers already present over western Colorado
and eastern Utah on visible satellite and radar. Model lapse rates
further support this outcome with values forecast to be between 8
to 9 degrees C per kilometer late this afternoon and into the
early evening. Model trends have continued to increase total QPF
values throughout the day with the HRRR now showing maximum QPF
values in Clear Creak and Summit Counties of around an inch. I
continue to believe these max values are overdone but there is
good reason to think QPF values will be higher than originally
expected. Therefore, I have increased POPs, QPF, and snowfall
amounts such that areas above 10,000 feet can expect 2-5 inches of
snow south of Grand Lake through north Park County. With these
convective showers it is of note that the snowfall rates will be
higher than normal and may be between 1 to 2 inches per hour under
the heavier showers. Travel in this area above 10,000 may become
briefly hazardous from 6pm to midnight.

Flow turns northerly on Friday as the trough departs with gusts
up to 25 mph across the eastern plains tomorrow. Despite the
northerly flow, temperatures will only be a couple degrees
cooler.

.LONG TERM...(Friday night through Thursday)
Issued at 109 PM MDT Thu Oct 18 2018

Friday night upper level high pressure centered over the Pacific
Northwest will begin to move southwest toward Colorado. Flow will
initially be from the north-northwest on Saturday transitioning to
southwesterly flow as the ridge flattens Sunday afternoon. This
flow pattern will lead to warm, dry conditions through the
weekend. Temperatures are forecast to be well above seasonal
normals by Sunday.

Monday mid level moisture is expected to increase as a trough advances
to the east. This will allow for the advection of tropical
moisture into the region in southeast flow. Late Monday into
Tuesday ascent will increase slightly allowing for a slight chance
of showers over the high country and northern border. The
moisture is forecast to spread onto the plains Tuesday afternoon.
This would give most of north central and northeast Colorado a
slight chance of showers with the best chance remaining over the
high country. Warm temperatures will keep rain over the plains
with snow levels above 9,000 feet.

Wednesday the trough is anticipated to progress east as subsidence
forms across the forecast area. Thursday a front will move into
northeast Colorado with the ECMWF showing a secondary trough while
the GFS brings in only a weak trough at the end of the period.
There will be a potential for light orographic snow showers over
the mountains with dry conditions across the plains.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 831 PM MDT Thu Oct 18 2018

VFR conditions will continue through Friday evening. Southerly to
southeasterly winds at KDEN and KAPA should persist through the
evening, strongest at KDEN.

High res models continue to show weak showers pushing off the
mountains through 06z and weakening further as they move
eastward. These could survive enough to briefly drop ceilings at
BJC and APA but no major impacts are expected.

&&

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

$$

UPDATE...Cooper
SHORT TERM...Danielson
LONG TERM...Sullivan
AVIATION...Cooper

NWS BOU Office Area Forecast Discussion