Winter Storm Watch: Major Ice Storm Possible Central/East Metro/Gorge

Noon Monday…

The Portland NWS has issued a Winter Storm Watch for many areas west of the Cascades for Tuesday.  A WATCH means conditions are set up for a certain event to occur.  These will likely be converted to Freezing Rain Advisories and Ice Storm Warnings (depending on ice accretion expected) this afternoon.

A few thoughts:

1. A major and possibly crippling ice storm is POSSIBLE in the central/east metro area similar to the 1996 event starting tomorrow and continuing through sometime Wednesday AM/midday
2. The Columbia River Gorge (all of it) is LIKELY to see a major/crippling ice storm tomorrow through Wednesday.  This on top of heavy snow/ice already could lead to roof collapses in that area.  Get roofs cleaned off today if you have the ability to do so.
3. Most of the rest of the metro area will LIKELY see at least some freezing rain Tuesday before temps creep above freezing, not a big ice storm but enough to cancel schools and turn roads icy for a good chunk of the day.

HIGHLIGHTS

• The low-level airmass doesn’t change at all through Tuesday AM as frigid air keeps pouring out of the Gorge.  We remain below freezing in most of the Portland area.
• Rain arrives sometime between 7am-11am Tuesday, freezing on contact in almost all areas in the western valleys of Oregon/SW Washington
• Roads in most of the metro area (that aren’t already icy) will turn icy within 15 minutes of the rain arrival
• Light freezing rain continues through the afternoon as temperatures rise to near/above freezing south and west metro.
• Temps remain in the 20s East Portland to around the Gorge entrance…solidly frozen all day there.
• Heavy rain arrives around sunset and it pours all night long into Wednesday AM as a brief “pineapple express” parks itself overhead
• West and south metro should be slightly above freezing during this time, thus avoiding a big ice storm.
• DURING THIS PERIOD, EAST PORTLAND OUT TO THE GORGE LIKELY REMAINS BELOW FREEZING
• If models are correct, 1-2″ ice could form on all exposed surfaces, doing tremendous damage to trees/powerlines that we haven’t seen in 20 years.
• Any precipitation through Wednesday midday/afternoon should be freezing rain in the Gorge.  1-2″+ is likely out there.

COMMUTERS/SCHOOL

• I’d say both should be skipped tomorrow
• Check right away in the morning when you get up to see how temperatures are doing and where the precipitation is of course.  Maybe I’ll be totally wrong this time and we’ll get lucky!

The main reason for the forecast change in the past 24 hours isn’t just the usual “oh, the cold air is sticking around longer than expected”, it’s because models have backed off on bringing lower pressures to our north.  Surface low pressure centers are expected to stay much farther offshore now.  This only gives us weak southerly flow up the valley and not until Tuesday night for central/south valley.  Since we have the low-level cold air in place and it is VERY cold, that means a longer period of time spent below freezing central/east metro.  The preferred ECM

 

WF model shows the progression pretty well, 4am Tuesday wind…strong easterly gradient and we’re in the deep freeze

Then 4pm Tuesday…still 7 millibars easterly gradient (cold east wind still sending cold air into metro area).  Only a very light southerly flow up central/south valley.  We’re still frozen most of metro area at this point.

Wednesday 10am, southerly wind has taken over south of metro…easterly STILL going central/east metro.  This would keep central/east metro frozen through midday Wednesday!

And the really bad news.  Check out how much rain has fallen during that period, a solid inch to two inches glaze ice over the below-freezing areas.  There is no loss of precip in this case from the dry Gorge wind as we’ve seen in snowstorms because that dry air is very thin, in the lowest 1,500′ of the atmosphere:

That’s a major ice storm that we haven’t seen in many years central/east metro and the Gorge.  The WRF-GFS model’s rain forecast is similar, showing 1-2″ rain in the metro area by Wednesday afternoon.

Wednesday evening a cold front passes overhead and we change to a showery pattern.  Then temperatures even in the Gorge will gradually rise above freezing and in the metro area too.

Is there anything you can do to prepare?

• Get plenty of gas for your generator if you have one of those and live in the ice storm area
• Watch for falling trees/limbs Tuesday night and Wednesday AM.  I don’t expect significant icing until after dark Tuesday when the heavier rain arrives

Chief Meteorologist Mark Nelsen