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April 2018

Biggest Warmups Following A Snowstorm

Clipart_meltingsnowman2

 

Less than 12 hours after the the last snowflakes fell from the 8.4" snowstorm of March 21-22, 2018, the temperature rose to 50° later that afternoon.  This was the greatest rebound in temperature following a snowfall of six inches or more since 1915 (when the high reached 51° the day after a snowfall of 10.2" on April 3).  Looking at lesser snowfalls (between two and six inches), there have been six that were followed by even milder temperatures, with the the warmest reading being 58° the day after a snowfall of 2.8" on March 13, 1943.  (Coincidentally, in March 1964, on the same dates as this year's 8.4" snowfall, a high of 50° was also reached the afternoon following a snowfall of 4.9".)  And looking at major snowstorms (accumulations of a foot or more), the biggest warm-up came after the 16-inch snowstorm of Dec. 20-21, 1948, when the high reached 42° on the afternoon of the 21st.

 

Going back to 1900, there have been five snowfalls of six inches or more that have been followed by a high of 45° or milder (either during the afternoon of the day of the snowfall or, if snow continued in the PM hours, the following day).  By contrast there have been twenty snowfalls between two to six inches that have been followed by temperatures of 45+.

 

Chart - Warm-up after Snowstorms

 Chart - Warmup after snow

 

Previously, I posted an analysis that examined significant snowfalls that occurred the day after mild temperatures.  It can be found here.

 

March 21 snowfall
Snowy day in SoHo (March 21, 2018)

Biggest Snowfalls In Above-Freezing Temperatures

Spring-snowstorm-CentralPark-NYC-02April2018

 

In high school science class we learned that snow falls in temperatures of 32° or colder; however, that's not always the case as accumulating snow has been known to fall in air temperatures above the freezing mark - as was experienced three times during the winter of 2018 when 4.4" fell on 2/17, 3.2" on 3/7 and 5.5" on 4/2 (and 5" on 3/3-4 in 2019).  This phenomenon happens when snowflakes fall through a shallow layer of air close to the surface that's a few degrees above freezing.  If the air is relatively dry, some of the flakes will evaporate, cooling the air enough to keep the flakes from changing to rain.  (This is a simplified explanation; a more in-depth discussion on the topic can be found here.)

 

During the past 100 years four days have had snowfalls of four inches+ on days with temperatures above freezing, six have had three inches or more and 17 days have had accumulations of two inches or more.  The 5.5" snowfall on April 2, 2018 has the distinction of being the greatest amount to fall with temperatures above 32°, breaking the previous record set 60 years earlier (4.7" on March 22, 1958).  Perhaps the reason so much accumulated was due to the fact that it fell heavily in a short period of time (five hours), thus hindering the rate of melting.  And as mentioned in the opening paragraph, the winter of 2019 saw a five-inch snowfall on March 3-4 in which the temperature got no lower than 33°.

 

33 degrees

 

Please note that this analysis is limited to days that had temperatures above freezing for the entire day.  Days in which snow fell when temperatures were above freezing but also had temperatures of 32° or colder for part of the day aren't included since hour-by-hour  precipitation is provided for the liquid amount that fell rather than snow.  (However, an exception to this limitation is the snowfall on 2/17 of 2018, when the high/low was 40°/28°, since I actively monitored the hourly temperatures and, therefore, was able to verify that no snow fell during the hours in the morning when temperatures were below freezing.  Instead, 4.4" accumulated in the evening/night when temperatures were above freezing.)

 

Chart - snow in above freezing temperatures

 

The coldest temperature for the dates on the above chart were either 33° or 34°.  However, there were a few dates with smaller accumulations in which the coldest temperature was 35°: 3/17/1965, when 1.1" was measured; 3/9/1955/0.1"; 2/15/1950/0.4"; 1/9/1932/0.2" and 4/2/1927/0.1".

 

Slushy snowstorm


March 2018 Weather Recap: Chronic Chill Follows Mildest February

Slushy new york march 2018For the second year in a row March was colder than February (both which were the mildest on record), a rarity that occurred just once before, in 1890 and 1891.  March 2018 will also be remembered for the four nor'easters that lashed the area in a three-week period as well as a 22-day streak with below-average temperatures.  This was the fifth March of the past six that was colder than average.  However, despite the chronic cold there were no Arctic outbreaks; the coldest low was just 27° (the coldest reading in March is typically in the upper teens) and the coldest high was 39° (the typical coldest high is around 32°).

 

FOUR NOR'EASTERS IN QUICK SUCCESSION

Nor'easter #1 (March 1-2) - This was a rain event, with 2.24" measured.  Colder air moved in as the storm exited in the early evening, briefly changing the rain to wet snow, but it didn't produce a measurable accumulation.

Nor'easter #2 (March 9) - This storm produced wet snow which accumulated 3.2" in the afternoon and evening.  Despite a prediction of 8-10" the accumulation was kept down because temperatures stayed above freezing.  Total liquid precipitation was 1.41".

Nor'easter #3 (March 13-14) - This was the least consequential of the storms.  Despite a prediction of significant snowfall there was no measurable accumulation, and just 0.36" of rain.  Curiously, Central Park was the only reporting site in the metro that didn't have measurable snow (JFK had 1.2" while LaGuardia and Newark both reported 0.3").

Nor'easter #4 (March 21-22) - This was the only storm of the four in which the temperature was 32° or colder for much of the duration.  As a result, a significant amount of snow, 8.4", piled up, making it one of the City's ten largest snowfalls after March 15. 

All but 0.11" of the month's 5.17" of precipitation came from these nor'easters.  This was the second month in a row with more than five inches of precipitation, the first time it happened since Dec. 2014/Jan. 2015.  The combined 11.00" that was measured was close to 50% above the average amount of precipitation for the two months (7.45").

 

MORE CHILLY THAN COLD

The 22-day streak with below average temperatures (March 7-28) was the fifth longest in the years since 1950.  But while it was one of the longest below-average streaks on record it wasn't a particularly cold one.  At 4.5 degrees below average it was the least below-average of any streak of 15 days or longer.  Another curious aspect about this March was that it shared some characteristics with Marches that were milder than average, i.e., very few lows in the 20s (just two), a coldest high that was around 40°, and no days that were ten degrees or more colder than average. 

Overall, March 2018 was 2.4 degrees colder than average, which was 1.8 degrees colder than February.  By comparison, March 2017 was 3.3 degrees below average and 2.4 degrees colder than February.  Typically, March is 6.2 degrees milder than February.

 

Chart - most days in a row below average

 Chart - 5 of 6 marches cold

 

Other March recaps: 2017 and 2016