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April 20-21 late season snow potential


Hoosier
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13 minutes ago, sbnwx85 said:

Late season lock it in for South Bend, please. Last year we had about 3” on 4/17/20 and I thought that was really amazing. A beautiful, wintry scene around 11 am. Then it looked like spring again by 4 pm. 

 That sort of happened here on April 15th last year. Very heavy burst of snow midday dropped about 1.5" for an instant Winter wonderland. By evening it was back to Spring. On April 17th however, the 3.4" that fell was an absolute Winter wonderland in the morning and it did turn slushy late in the day however we still had about 2" left that froze solid. So the morning of the 18th you could walk on 2" of ice crust . By afternoon however Spring had returned

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For late April...the upper level cold pool looks very impressive with this event. The source region of the air mass comes from the Siberian side of the pole with 250mb flow vectors pointing due south from Nunavut. A wave then develops along the thermal boundary which is how we get our snow swath.

gfs_uv250_nhem_12.thumb.png.9f325db0c886063971edaf1bb840a14f.png

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A key piece looks to be the leading Quebec vortex which really helps determine where the thermal boundary sets up behind. If the vortex digs further south, the confluence zone also ends up further south and you get more of a GFS like track. If the opposite occurs, there's more room for the great lakes energy to amplify further NW.

gfs-deterministic-conus-vort500_z500-8963200.thumb.png.c12d2c284d9c94741c48b11683551f94.png

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33 minutes ago, Chicago Storm said:

OP GFS continues to be on the SE side of guidance.


.

Ukmet even further SE meanwhile 12z canadian is in the middle, 6-10 for most of SE MI. Id expect a little more waffling until the front settles SE the next day and a half. The low looks to be strengthening and maxing out in the region which is nice to see. Recently, more often than not, low pressures would be fizzling as they approach dtw, due to the darn block and other factors.

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This is sort of a pick your poison thing, but from a snow side, it does appear that some areas will drop below freezing while it is still snowing, especially as we get toward Tuesday evening and certainly after.  This would help make the snow not quite as wet/heavy as earlier on, thus reducing the weight.  On the other hand, dropping below freezing means more hours spent below freezing and increases the chances for hard freezes.  

I expect temps to be AOA freezing here for virtually the entire snow event, as onshore flow keeps things a bit warmer.  Then a drop well below freezing as flow turns more offshore, so I may get the worst of both worlds lol

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Could argue that the farther southeast it goes, the more historic it gets.  In any case, I think we are locking in on an event of that magnitude for somebody.  A reasonably safe bet would be that the final outcome ends up somewhere in between the current NAM and UKMET runs.

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This is a major key to this event and helps to reinforce confidence that we are on the verge of seeing a pretty anomalous outcome.  Here are the progged GFS 925 mb temps... and I would point out that other models are even a little colder.

 

12Z-20210417_GFSMW_925_temp-72-96-50-100.gif.55537cb08ef339ea09a47eec32e819d5.gif

 

If 925 mb was barely below freezing, it would be very problematic this late in the season.  But this is solidly below freezing at only a couple thousand feet up, which will enable precip type to be snow and help knock surface temps back toward freezing in the main band of precip.

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8 minutes ago, StormfanaticInd said:

Ukmet image.thumb.png.88f9a5bf3d15f3761c03fe704d5da38a.png:blink:

It's almost hard to put into words how rare the UKMET would be for Indianapolis.  Indianapolis has had a total of 7 calendar day snows of 1"+ after 4/15, with the biggest being 2.5".  This model is currently a southern outlier though and I wouldn't put a ton of stock in it at this point.

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1 minute ago, Hoosier said:

It's almost hard to put into words how rare the UKMET would be for Indianapolis.  Indianapolis has had a total of 7 calendar day snows of 1"+ after 4/15, with the biggest being 2.5".  This model is currently a southern outlier though and I wouldn't put a ton of stock in it at this point.

I'm not but just the fact that its even showing this possibility at this close range is crazy. Lol. Hopefully its wrong :gun:

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6 hours ago, IWXwx said:
IWX is indeed all in:

SPC UPPER AIR SOUNDING CLIMATOLOGY, CIPS ANALOGS AND GEFS M-CLIMATE RETURN 
INTERVAL ALL SUPPORT AN EVENT THAT MAY POSSIBLY BE CLASSIFIED AS A
50 YEAR OR 100 YEAR EVENT. A VERY STRONG ANOMALOUS SIGNAL WILL SET
UP TUESDAY. HAVE SIGNIFICANTLY UNDERCUT MODEL BLEND GUIDANCE GIVEN
DIABATIC PROCESSES TUESDAY INTO TUESDAY NIGHT. TEMPERATURES SHOULD
FALL DURING THE DAY TUESDAY WITH RAIN CHANGING TO SNOW. THE ECMWF
WHICH IS A LITTLE SLOWER THAN MOST OTHERS, REALLY BURIES NORTHWEST
INDIANA INTO LOWER MICHIGAN WITH 6" TO 10" OF SNOW. THE 2-DAY
RECORD SNOWFALL DURING THE SECOND HALF OF APRIL IS 9.9" IN 1961.
CURRENT THINKING IS 4 TO 8 INCHES OVER NW INDIANA WITH LESSER
AMOUNTS SOUTHEAST.

Looking back at that 1961 event, I am pretty sure the leafout was not as far along as now.  It's interesting to note the reports in Storm Data from back then, which mention drifting snow with even some 10 foot drifts.  That is crazy for mid-April in Indiana.

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