textproduct: Boise

LONG TERM...Monday through Friday

An upper Alaskan low is expected to steer eastward across the upper Northwest and Northern Rockies Monday, bringing a 20-40% chance of precipitation mainly over higher terrain. Northwest flow from this low passage is expected to drop snow levels down into the 2000-3000 ft range for the mountains, favoring the potential for snowfall. Valley daytime temperatures are expected to drop into the mid to upper 40s Monday through Wednesday. Long-range ridging is expected to build behind this low, but there is slight guidance uncertainty over the steering of an oncoming moisture band off the Pacific NW coast; precipitation remains possible for higher terrain on Tuesday. Wednesday through Thursday will also see chances of precipitation throughout the area, with snow remaining possible over higher elevations as snow levels are expected to trend up into the 4000-5000 ft range and into the 6000 ft range for lower elevations. Valley daytime temperatures are expected to take on a slight upward trend into the upper 40s and lower 50s Thursday through Friday. Late Thursday into Friday will see considerable guidance uncertainty over the arrival of either a longwave trough off the NW or a closed upper low digging into the area.

AVIATION

VFR/MVFR low stratus continuing to lower overnight. Dense IFR/LIFR fog widespread over sheltered valleys with light winds. Conditions improving Friday afternoon, but may redevelop Friday night. Surface winds: light and variable, except locally up to 10 kt from the N-NW overnight. Winds aloft at 10kft MSL: variable less than 10 kt.

KBOI...LIFR fog/low stratus expected to continue likely dissipating by early afternoon. Surface winds light and variable.

Weekend Outlook...Areas of valley fog/stratus Saturday/Sunday mornings with improvement each afternoon. A cold front on Sunday will bring light precipitation mainly to Northern areas and mountain obscuration late Sunday night into Monday. Snow levels 5500-7000 ft MSL. Surface winds: SE and variable under 10 kt Saturday and early Sunday, and SW-NW 5-15 kt late Sunday night into Monday as the cold front moves through.

AIR STAGNATION

High pressure will support stable conditions through Sunday night, with daytime mixing heights peaking near or slightly above 2000 feet AGL. Light winds and strengthening near-surface inversions will result in relatively poor ventilation in the valleys. A cold front will arrive late Sunday into Monday and lead to improved mixing and ventilation.

BOI WATCHES/WARNINGS/ADVISORIES

ID...Dense Fog Advisory until 10 AM MST this morning IDZ012-014-016. OR...None.