How to decode this METAR published during huricane Harvey?
I suggest changing the title, else this question is of relatively limited interest: the METAR message for KNGP has been already updated with new values.



How to set the altimeter when the "sea level pressure" information is not available?


ATC will be able to provide the information. When aerodrome QNH is not available, the Regional Pressure Setting is used ("safe" setting for the highest location of the region).


6////: Group 6RRRR, 3- and 6-hour precipitation amounts


3- and 6-hour precipitation amounts are missing, sensor not operative or not reachable.


402610239: Group 4snTTTsnTTT, 24-hour max/min temperature in tenths degrees


Maximum temperature in the past 24 hours + 26.1 °C.
Minimum temperature in the past 24 hours + 23.9 °C.


53014: Group 5appp, hourly pressure tendency and and actual change in tenths hPa


Atmospheric pressure now higher than 3 hours ago. Decreasing or steady, then increasing; or increasing then increasing more rapidly.
Variation is 1.4 hPa


PWINO: Sensor status indicators (RVRNO, PWINO, PNO, FZRANO, TSNO, VISNO_LOC, CHINO_LOC)


Precipitation identifier information (rain or snow) not available.


See this decoder and this list of METAR abbreviations.