Mt d'Arbois - 06/12/19

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Mt Blanc Valley Weather - 3 Day Forecast

High 1031hpa, Azores. High 1030hpa, Black Sea. Slightly lower high pressure ridge stretching in‑between, still protecting the Alps this Friday morning. Very light flow high up, between north and west. Barely deepened altitude low however, northeast of Canary Islands to Gulf of Genoa: while merging with polar air thalweg from Norwegian Sea, it could give some activity to resulting cold front which is expected to crossing over our Region southeast bound overnight. This means ending of current fair weather spell, therefore. Condition expected to getting noticeably worse by Monday afternoon, upon crossing of an occluded front (linked to <950hpa low north of Scotland) which might trigger fairly heavy snowfall, blown by severe gale force winds high up during the following night.
TODAY - FRIDAY DECEMBER 6
Fair weather ‑ cold in the valley at first‑ mild on sunny slopes later
SKY CONDITION : clear until the latter part of the afternoon ‑ thin high cloud from northwest in the evening. Period of sunshine close to 95%.
PRECIPITATION : none.
WIND - ground level : calm ‑> SW very light.
WIND - mid mountain : WSW very light ‑> light to moderate in the evening.
WIND - high mountain : NW very light ‑> WNW moderate in the evening.
TEMPERATURE : low ‑5C - high +6C - temperature inversion toward 1600 m early in the morning.
ISOTHERM 0C : 2400 ‑> 2100 m.
ISOTHERM -10C : 4100 ‑> 3900 m.
OVERNIGHT
Cloud crossing ‑ shower risk, latter part of the night ‑ strong wind high up later
SKY CONDITION : high cloud thickening soon after nightfall, then continue to grow while getting broken towards the latter part of the night.
PRECIPITATION : shower risk towards the latter part of the night ‑ rain‑snow limit towards 1700m.
WIND - aloft : WNW moderate ‑> NW strong.
TEMPERATURE : low +1C.
SATURDAY DECEMBER 7
Frequent cloud crossings ‑ morning shower risk ‑ windy high up, decreasing
SKY CONDITION : mostly cloudy ‑ frequent patches crossing over ‑ becoming cloudy with sunny spells in the afternoon. Period of sunshine close to 45%.
PRECIPITATION : shower risk in the morning‑ rain‑snow limit towards 1700m, rising trend.
WIND - aloft : NW strong ‑> moderate.
TEMPERATURE : low +2 C - high +5C.
SUNDAY DECEMBER 8
Cloud crossings ‑ windy high up ‑ afternoon shower ‑ quite milder than seasonal average
SKY CONDITION : cloudy with sunny spells ‑ mostly cloudy at times. Period of sunshine close to 40%.
PRECIPITATION : showers in the course of the afternoon, light to moderate on the whole ‑ rain‑snow limit towards 1700m, dropping towards 1200m ‑ expected snowfall towards 1800m: 5cm.
WIND - aloft : NW light to moderate ‑> > moderate to strong.
TEMPERATURE : slightly dropping lows ‑ rising highs.
EXTENDED OUTLOOK UNTIL WEDNESDAY DECEMBER 11 2019

Monday: very unsettled ‑ continuous precipitation, becoming moderate to heavy later ‑ rain‑snow limit towards 1000‑1100m ‑ northwest to north reaching severe gale force high up in the evening. Tuesday: mainly cloudy ‑ snow shower (800m) slight risk ‑ morning north gale high up, backing and decreasing later. Wednesday: strong westerlies keep blowing moderate snowfall high up.

Forecast reliability : fairly good until Saturday ‑ poor thereafter.