JACK FROST IS IN THE HOUSE...

23 hours ago

5 min read

We haven't had to deal with Jack Frost and his icy touch more than a handful of times this year. However, Friday night the situation was prime for frost. In fact, it was one of those situations where old Jack could end up tossing in a freeze for good measure. A freeze occurs when surface air temperatures drop to 32°F (0°C). North of I-80 that benchmark has been achieved. South of I-80, it's been a different story where lows have been no colder than 33-37. Even where readings have reached 30-32 in the north, little if any impacts were noted with vegetation still flourishing due to minimal time spent at those temperatures. Here's some background on frost and freezes.

Key Freeze Classifications

Light Freeze: Temperatures range between 29°F and 32°F (-1.7°C to 0°C) for a few hours; it typically damages or kills tender annuals and warm-season crops (like tomatoes and peppers), while hardier vegetables survive.
Hard Freeze: Surface temperatures reach 28°F (-2.2°C) or lower for an extended period, causing ice to form inside plant cells and ending the active growing season for most vegetation.
Freeze vs. Frost: A freeze is an air-temperature event (always measured a few feet above the ground), whereas frost refers to the actual formation of ice crystals on surfaces from frozen dew or water vapor, which can sometimes happen on calm, clear nights when air a few feet above ground is slightly above 32°F.

Friday nights situation is cut and dried in the sense we initially have clear skies, light winds, and dry air. That's the 3 amigos you look for when it comes to achieving maximum radiational cooling. Where things are a bit gray is whether or not some warm advection clouds arrive soon enough to stop the fall of temperatures. Readings fell sharply early Friday night reaching freezing levels in most areas by 2:00am. After that, several models depict a band of high clouds arriving from the NW that could act like a blanket stopping the plunge. The HRRR shows the clouds entering the region around 4:00am.

If that ends up being the case, it could trim away 2-3 hours of additional cooling. That would hold lows more in the range of 30-32. If the clouds are slower, or don't materialize, that would drop readings more into the range of 26-30. Either way, it's going to be cold and frosty by early Saturday. If you still have vegetation and plants you care about, hopefully you covered them up. Here's what the HRRR is depicting for lows with clouds arriving late.

OUT WITH THE COLD

Thanks to the cold start Saturday, it will take the dwindling November sunshine some time to do its thing. Eventually, with partial sunshine highs should fight their way back into the upper 40s, maybe 50 in the southwest. Sunday, southerly winds and a building ridge pump Pacific air back into the pattern with highs topping out around 59 north to 64 south. Monday should be even better with highs of 62-67. As you can see below, those readings are 14-15 degrees above normal.

We should be able to squeeze out another mild day Tuesday with similar highs. However a back door cold front is on the move and that starts to chip away at the warmth Wednesday, especially across the north where highs will hold in the low 50s while the south still enjoys readings of 58 to 62. Thursday and Friday with the front through the region, everybody takes a chill pill with highs in the 40s areawide.

A new trend that's popped up is a tendancy for the chilly air to linger in the long term. The EURO starting November 12th has 9 out of the next 10 days in the low to mid 40s in the Quad Cities. That's far cooler than what was shown 24 hours ago. I'm really not sure I buy into that just yet. I think it's at least 2-3 more weeks before the doors really open to much in the way of cold air locally.

The colder air (if it happens) shows up in the temperature departures for November 14th. East of the Rockies everybody is well below normal.

This could be the beginning of a pattern realignment that teleconnections have been hinting at for some time. I won't get into it much other than to say blocking is expected to strengthen over southern Canada. That allows colder (more seasonal) air access to the country. Even more important is the evolution of the sub-tropical jet which becomes highly energized and amplified across the nation. Seasonal forecasts have this becoming the dominate pattern for winter. Here's the mean storm track on the EURO at 500mb for December, January, and February.

Along with this upper air pattern, the AO (Arctic Oscillation) and the NAO (North Atlantic Oscillation) are shown to be predominantly negatve from now through December 22nd. That would serve to produce active and at times stormy weather over the central and east. It would also promote near to perhaps slightly below normal temperatures, especially after November 22st or 23rd.

The AO

Another factor which is much more difficult to measure, is The Polar Vortex breaking down weeks ahead of schedule. Stratospheric winds are dropping to near zero and are forecast to reverse in the coming days, making this the earliest Sudden Stratospheric Warming (SSW) on record. We usually monitor the overall progress and strength of the Polar Vortex by just looking at its wind in the stratosphere. A strong positive (westerly) wind indicates stable conditions or a strong Polar Vortex, but weak or even negative (easterly) winds indicate a complete reversal and a breakdown of the Polar Vortex.

All EURO and GEFS ensemble members show a breakdown and wind reversal at some point soon, making it very likely to happen. It also looks brief, with the winds recovering by mid-November. But what makes it very unusual is its early timing and a full reversal comes nearly a month before any past event, as seen in the image below that compares to past SSW events.

The bottom line is that when temperatrures rapidly warm around the stratospheric Polar vortex, it slows, reverses, and weakens dramatically from it usual strength. In turn, the weekened vortex allows the cold air that is usually contained in the Polar Vortex to descend and move south into the mid latitudes of the Midwest.

With such a strong early El Nino and an unprecedented strat warm, its hard to say what the impacts will be locally. However, after a strong early November collapse research does indicate that for many areas there is an increase in December Snowfall.

December temperatures also have a tendancy to be colder.

January snowfall also increases during an early November Polar Vortex disruption as shown below.

January temperatures also show a strong correlation to be colder over much of the nation.

Anyway, the general idea of the EURO winter outlook is for near to slightly below normal temperatures and snowfall that should end up normal to perhaps slightly above. Of course, we are in uncharted territory with a record strong El Nino attempting to offset the unique strat warm, -AO, and -NAO. It will be very interesting to see if modeling can resolve the chaos accurately. As it stands now, the latest means of the EURO weeklies show this for snow between late November and December 21st. This has been consistently shown on the EURO for the last week. As always, we shall see